OK so now you found a knife that appeals to you. Is it a quality piece, is it worth the price being asked? Weather you collect with an eye toward investment or knives for use or for the artistic appeal of the knife the elements of design, fit, finish and feel are the keys to assessing quality. Here is what to look for;
Design
Visual appeal is the first step in enticing you to examine the piece further. Though subjective many elements of visual appeal; the lines, flow, colors and materials, are universal to any knife.
Look for an artistic flow to the knife. Make sure that everything goes together; the parts of the knife, the lines, shapes and angles should work together creating a pleasant whole. The blade to handle ratio must look right.
A quality knife combines successful execution of functional design with artistic creativity. Put another way, the knife should serve the purpose for which it was intended and it should also be pleasing to the eye.
The materials used to make the blade, handle, fixtures and decorative add-ons, if any, should be of good quality.
Blade steel, be it a monolithic steel composition or a Damascus patterned steel, has a great impact upon the overall appearance of the knife. I will, at a macro level, with respect to design, address it as such. At a micro level the qualities of the various singular steels (ATS34, D2, RWL34, SV30, CPM154, etc..) are pertinent to the intended use of the knife as relates to stain resistance, sharpening ease, edge retention, etc.. I will not address specific steels as they relate to quality, as their choice is often subjective and task specific. The singular steel compositions provide for a clean steely/gray appearance of the blade and offer up several finish options. Damascus blades offer an altogether different set of appearance options. Damascus patterns can be works of art in there own right, they can be random or specific patterns, they may contain highly detailed designs, they can receive different finishes and etches offering a myriad of looks to the finished knife. Considering two identical knives with the only difference being a Damascus blade, the Damascus, being more difficult to create demands a premium. However, a Damascus blade alone does not warrant a premium or represent higher quality than a knife without a Damascus blade because the value is a sum of the whole, not just the blade. What I am saying here is, does the blade go with the overall design of the knife and is it finished well.
We have touched a bit on blade material but it is necessary to go beyond that in considering the overall design of a knife. The handle, bolsters, scales, screws, spacers; are all other areas of a knife that comprise the overall design. Typical materials used can be synthetic (G10, Carbon Fiber, Micarta, Titanium, etc..) or natural (wood, ivory, stag, pearl, gold, etc..). As with Damascus, generally being more difficult/expensive to obtain, certain materials demand a premium over others; ivory, quality pearl, decorative jewels, etc.. Again, materials alone do not warrant a premium or elevation of quality, the value is a sum of the whole.
Decorative elements of a knife can be a key design consideration enhancing value. Materials (gold, pearl, precious stones, etc..) as discussed below are part of this equation. A key to their increasing value is their appropriate use in the overall design. File-work is an often-used decorative design element. File-work should be scrutinized technically for crispness and uniformity and aesthetically for creativity and appropriateness to the overall look of the knife. Engraving can be used to enhance the visual appeal of a knife and can up the quality quotient if well executed.
A well-designed knife is more appealing than an equally well made knife with poor design and therefore of higher quality.
Fit & Finish
Blade grind is a key element in evaluating a knife. Grind lines should be even and crisp. When viewing a knife with the cutting edge towards you the grind lines should match as they come down from the blade. Also, when looking at the tip the grinds should be the same and the blade should be straight and the clip should match on both sides.
Blade finish is a great indicator of the skill level of the maker. Weather hand-rubbed or machined the finish should be smooth and consistent with no scratches or marks of any kind. Look for uniformity; a change in color or shade may indicate a poor sanding or polishing job. A hand rubbed finish with a fine grit consistency is more desirable than a machined finish. With respect to Damascus blades, it should be free of flaws in the welds and the pattern and etch should result in a clean crisp finish.
Handle the knife to check for proper finish. There should be no sharp edges were none are intended. There should be no gaps between the bolster(s) and handle or liners and handle. Screws and pins should be flush with handle or bolster except in the case of domed pins, which should be round, smooth and symmetrical. On a knife that is pined the pin should blend with the bolster and be indistinguishable.
Folders
The blade should be centered with respect to the handle/frame when closed.
On a folder the blade should not wobble up and down or side to side when open. Acceptable play in the fit of all the moving parts should be very close, topically less that a half thousandth. A human hair is about .0035, (that's 3 and a half thousands as a reference).
A folding knife should open smoothly. A knife with a pivot mechanism should open with very little friction. On knives utilizing a detent (liner-locks) the blade should click solidly into the detent. When closing a liner lock the ball detent should pull the knife closed in that last fraction of an inch. The amount of pressure to overcome the detent upon opening is a personal preference but once overcome the opening should be smooth.
With a slip-joint the resistance in opening and closing the action is a personal preference, check to make sure it is comfortable for you. Make sure the knife “walks and talks”, i.e. slides smoothly and snaps open.
In a lock-back knife the space between the top rear of the blade and the lock-bar/spine of the knife should be almost seamless. On the sides there should be the slightest of clearances so that the blade does not rub against the frame when the knife is opened and closed (this will cause galling and mar the finish). The better the tolerances the less of this space you will see but it should be there.
Locking mechanisms need be secure. With a liner-lock or frame-lock there should be sufficient tension on the lock and the lock should be at a 90-degree angle to the rear of the blade to insure the lock will hold.
Edges should be consistent, either crisp sharp edges all around or, if a radius is used this should be uniform on all edges.
Pins or screws should have clean sharp edges and consistent finishes.
All areas inside and out should look finished on a top-end knife. Look at the front of the folded knife and as applicable to the knife design areas such as the stop, ramp or cam should have a clean crisp fit and finish.
Fixed blades
Handles should be symmetrical and centered with respect to the blade (unless intentionally designed otherwise such as with a palm swell or antler material).
If there is a guard it should fit cleanly to the handle and blade. There should be no solder lines or gaps between the blade, guard and handle. Depending on the design it should be symmetrical or at uniform angles and centered on the blade.
Feel
Balance is another key element in the feel of a knife. It is particularly important in a knife intended for use. While balance is a personal preference it is usually considered appropriate that the balance point rest around the guard or pivot point of a knife. The knife should feel right in the hand.
The weight of the knife is an important consideration in a using knife, not so much in apiece acquired for artistic value. Like wise, the feel or comfort in the hand is important in a using knife and also, I believe, is important in a knife weather intended for use of not. If you are going to handle a knife it should feel comfortable and right, if it is going to be on a pedestal and not touched then that is a different matter.
Cost as an aspect of quality
Some relate cost to quality and there is often validity in this. Think about it, what makes one knife cost more than another. Material is one factor; some materials just cost more than others. A perfect example of this is Pearl, which is available in different grades and types, the finer grades and rare types costing more. Damascus costs more than plain steel.
Bolsters and dovetails are more difficult to build than plain slab handles. A knifemakers name can also add to cost. With a famous maker you may pay more, however, the name often represents a known level of quality and/or historical significance or rarity. Provenance, established through a makers certificate of authenticity, a photo in a magazine or a history of a knife is another factor that can increase cost. The 'quality of provenance' usually makes a considerable difference to the value of artwork.
Summary
Each of these factors; design, fit, finish and feel will be present to varying degrees with a knife and these factors determine the quality of the knife. When it comes to custom knives quality and cost do not always exist is tandem but there is usually a strong correlation. Buy what you like but determine the quality and factor that into what you buy and how much you pay.