Machinist's levels will generally be very sensitive, detecting very small changes in level. Set it on your bench and place thin paper - card stock or printer paper - under it until the bubble is centered. It's handy to put down a piece of printer paper first, and mark around the level, for reasons I'll explain in the next paragraph.
While you've got the bubble centered, pick up the level and turn it end for end, carefully setting it down in the exact same place (that's why you marked around it in the first step). If the bubble's still centered, it is in fact a level; if not, it's a pretty little piece of metal with a bubble in it; making it into a level would require getting a replacement vial (the part with the liquid and bubble in it) and carefully replacing the old one, installing the new one so it's accurately level, not an easy job.
Now add one more piece of paper, and see how much the bubble moves. If it takes several pieces of paper before you see much movement, it's probably just a handy little level; if the bubble moves quickly, it's likely a machinist's level.