It seems a favoured trick to slice through printer paper perhaps curving the cuts ; it certainly proves the blade is sharp and the less noise the sharper the blade is. This certainly proves sharpness but whether it is the right sharpness is a different matter.
Another favoured way is to chip into the finger nail to me this is one of those sensations I abhor so I cannot comment too much on that an alternative is to hold a plastic pen at about 45˚ to the blade and let the blade fall across it if it sticks it is sharp.
Another is shaving hair .
These tests will prove a knife is sharp but may not prove it is the sharpness required for an EDC or the kitchen.
Another test which requires experience of touching many blades is the three finger test put your thumb on the spine of the knife and your fingers on the blade it is useful once your fingers know what to expect.
I think these two tests below are possible the most useful since if it is not your own knife you want to be certain that it will impress the recipient .
A good test for an EDC is to see if it will slice nylon/polypropylene line on a board if it slices through easily it has passed the test.
A good test for a go to kitchen knife is to see if it will cut through something packaged in a sausage like tube of polythene normally thick and the contents wants to be soft like processed ham , a soft cheese or a patê if you can slice it with ease it this means it will cut through most things in the kitchen.
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