golok :
Normally, there are tests that can be done in a planned situation but you and that guy are somewhere out in the field.
Since that is where knives are commonly used, obviously their performance can be evaluated in such a setting. What you would do would depend on the knife to a certain degree, though there would be large overlaps even amoung very different knives as they are all to some degree cutting tools. For example if he was holding up a fillet knife, you would look for sharpness, flexibility, cutting ability, handle ergonomics and security and corrosion resistance. Note how many of these would carry over into any type of knife.
Ideally you would want a large number of the appropiate fish at hand and all of the above could be evaluated just by processing the fish. However you are probably not going to have a few dozen salmon on hand when you are taking a walk so you will have to use what is at hand. This isn't difficult, after all we are talking about a very simple tool.
Flexibility can be tested by placing the tip against something rigid (a log / tree) and leaning into the knife seeing how easy it is to bend, and that it returns to true. Different people will want different levels of stiffness in their fillet blades so there will not be uniform ratings of this quality. Everyone however will want the blades to return to true.
Sharpness can easily be tested on very light vegetation such as grass. Take the blade and cut with it perpendicular to the grass and without a draw cut. The sharpest of blades can make this cut on even the lightest of vegetation. If you want to be quantitative you can use the degree of inclination you need to make the cut to assign a number to the sharpness (or the amount of draw).
Cutting ability can be tested by whittling a piece of wood, use hardwoods to test the very edge (angle and thickness), and soft woods to look at the properties of the primary grinds. If there are any soft fruits or other really thick vegetation (1-2"), this allows a look at overall blade cross section including thickness.
Handle ergonomics would already be evaluated in the above work, handle security by covering your hand in the fluids of the plants, or any other grease / lard you have on hand as fish can get very slippery after awhile. This is another area where personal preference comes into play, everyone will want a very secure grip, but ergonomics is very hand specific.
Edge retention is difficult to test on the spot as high quality knives can cut a large volume of material without needing to be sharpened. A good fillet blade for example can easily process dozens of large cod, so its not likely that a few trout will give you a meaningful estimate of edge retention or whittling a few pieces of wood. So you are going to need to do something else, artificial most likely. Use the edge as a scraper on hard wood for example to test resistance to deformation. The amount of deformation (and possibly wear) can be examined with light burnishing.
Sharpenability can easily be tested, it is simply the amount of work that it takes to restore a blade. So assuming you have a stone, blunt the blade by drawing it across a rock a few times and see how it sharpens.
Most of the above requires an experienced user who can judge the performance against his experience and can thus tell you what is good performance or just average, or some kind of benchmark is necessary. For example if you had two fillet knives you could easily rank comparatively if they were compared directly.
Of course other knives would require additional work, a large brush knife for example would want to be worked in hard wood to look at shock, soft wood for binding (both would combine for a cutting ability evaluation). Then look at limbing for control, and fatigue and see where the balance of power and endurance falls. You might also want to look at wood splitting and breaking (food / kindling), with and without a baton.
Sal :
What type of data would you like in place of the photo of the hair shaved from their forearm?
Cut something so slight that the influence of geometry is minimized, and rank according to load applied, or under a constant load, required edge length. This can be done to high precision (less than 5% uncertanity) with normal household materials, as can any aspect of blade testing. It is a very simply tool afterall, which people have been using long before we knew what electricity or computers were, and still the designs were refined and advanced.
-Cliff
