KenK, I do hope you mean your thumb NAIL ! Anyway your reply is the closest to what i use but i use a plastic pen and hold the knife at app. 45 degree angle to test the edge then reverse the pen & knife position so as to test "both" sides of the edge. Like the thumb nail it should not slip or its not sharp. Thing is I just recently learned this method from the internet and thus I'm not sure just how good it works so I thought i'd compare the methods here with this one.

No, I mean the fleshy part of my thumb. That's just how I've always done it. I don't push hard at all. Being that its my thumb, I can feel the feeling of the blade catching on the friction ridges of my fingerprint. I can really distinguish between slightly different sharpnesses.

Think of it like you hold the blade while shaving the arm, but its just on my thumb, which provides good feedback.

Thats the way I was taught from my father. You can really tell the difference in sharpness that way. You know its damn sharp when you go to do it like you always have and cut yourself. I now do different methods, but the thumb across the edge is my quick check on my EDC knives if they come into question...

Using a thumbnail to test an edge is pretty common. The actual thumb, less so.

One traditional Japanese test was to slice a slave or prisoner. A top quality blade was reported to be able to cut a man completely in half diagonally with a single stroke. I'm not sure if that is stronger than cleaving an anvil or sharper than slicing a dropped piece of silk but it is certainly a testament of toughness and sharpness.

Given that the police generally frown upon slicing live people in half I think such testing will recede into history and legend.

I use a few methods, one being the "does it shave" method - and different parts of the body take different sharpness

The other is slice paper, but the trick here is a hold a 8.5x11 sheet by one corner, and slice, pulling the blade so that every part of the blade gets tested. If there is a dull spot, the paper rips instead of cutting. BTW if you want to test for REALLY sharp, get thin "tissue" paper. Not the stuff you blow your nose on, but the stuff they use to wrap clothes when they are putting them in a gift box. The thinner paper makes it harder. Old phone book paper is also good, nice and thin

I saw an interesting treatease (sp?) on the difference between Japaneese and "western" swords, and the person pointed out that western sword makers COULD make swords that could be made as sharp, but they had a serious problem agains plate mail and even chain mail. The HUGE problem (and the whole reason for the diferential hardening of a traditional "Eastern" sword) was that they were VERY high carbon, and the edge had a real tendency to chip. In fact, he pointed out that if you look at REAL "Samurai" swords, you will almost always find chipping on the edge. This hardness issue lead to a different style of actual use, instead of "hacking" at your opponent, you tended to make a slicing cut, and the whole technique taught to use the sword is to make the sword do a pulling/slicing cut, and to lessen the initial "impact" of the blade on the target

Not to hijack the thread but the above is not entirely correct. It's true that the fighting style in Europe was somewhat different. There were many different styles of swords used in Europe, some highly specialized and some "all-purpose". Up until about the mid-13th c. most European swords were relatively thin, excellent cutters. Later on as armor improves, swords become longer and stiffer and there is more emphasis on thrusting.

Contrary to the popular belief, medieval European swords were for the most part quite light, no heavier than a katana per comparable blade length. Also, they were not meant to actually cut through armor. If you had to fight an armored opponent you'd ideally only use the point and target the weak spots in armor. But the sword was not in fact the primary weapon against armor, there were better, specialized tools for the job (mace, war hammer, pollaxe etc.).

The trick is, most European swords were either slack quenched or quenched and tempered to a relatively low hardness. In effect, the edge wouldn't last that long but the blade was very springy and tough.

The Japanese on the other hand never really figured out the tempering process. Hence they used clay and used a complex method of patternwelding, taking soft iron for the back of the blade and steel for the edge. A very similar process was known in Europe but was no longer used after the early middle ages because it wasn't cost-effective and you could get better results with more homogenous steel.

As a result, the edge on Japanese swords was fully quenched and very brittle while the back was very soft and tough - but not actually flexible because in effect it wasn't heat treated at all. It would bend under heavy stress rather than spring back. So metallurgically speaking, it was far from an ideal solution. The only reason why it worked is that the Japanese used lighter armor, largely lamellar made of hardened rawhide. A 15th or 16th c. Japanese sword would've been fairly useless against a European plate harness. Of course, it could still be used with great effect against an unarmored opponent. But so could a European sword from the same period.

Metallurgically, the very best medieval swords may have come from Iran and India. In those parts of the world relatively homogenous steel was already known in the antiquity, hence the famous Damascus blades. Billets of crucible steel were also exported to Europe. They were very expensive because of high production cost but could be forged into excellent blades.

For my really sharp blades, I like to see them cleanly cut a styrofoam cup without a lot of sawing motion. But those are generally only the smaller secondary blades on multi-blade pocket knives. Most knives just need to cut paper cleanly before I consider them sharp enough.

Sharp enough to roll cut completelythrough a piece of 12 oz leather I have. Another good test is to split shave the cover of a typical magazine so that I can pull a layer off without penetrating it completely, like splitting the thickness in half.

If I'm going for ultimate sharpness, I'll get a piece of newspaper or phone book, and hold it vertically in my left hand, from the top. With the knife in my right hand, I'll try to push cut straight down, measuring how far from the pinch grip my left hand has. Can't cut away from my left hand, has to be straight down, no slicing. My best is 4 inches.

If modern technology can detect the finest point of the edge, it is not sharp enough.
But seriously.
for an EDC knife i try shaving my arm. If, as is often the case, I've exhausted that method, i just use my thumb like KenK. For larger blades like a machete or sword i fell a small sapling. If it cuts through in one stroke leaving the cut face smooth, I'm satisfied.