If you do not have a tomato or piece of paper handy (which some reckon, will help blunt a knife designed for cutting other things) and don't want to shave patches off your arm (ditto) or risk slicing your thumb(nail) in two then it is possible to test the sharpness of a knife safely using only your thumb, as long as you remember this golden rule:

Never rub your finger ALONG the edge of a blade - rub it ACROSS it.

. The easiest way to do it is to turn the knife upside down then move your thumb across the blade once in a side to side motion. You are testing to see how thin that edge is where thin equals sharp. You should soon find it easy to gauge the sharpness of a knife without risk to life or limb.