For years I ground a way-too-shallow edge on my favorite chisel. It wouldn't hold up worth beans, so I dug through the junk drawer for something I could make a bevel gauge out of. What I came up with is this simple jig.
Just place the chisel's bevel into the angle formed by two plastic protractors and read the degrees on the protractor's face.
Cool idea! In case you don't have two protractors in your junk drawer, just cut the commonly used angles into a piece of plastic and use it as a bevel checker. If your tool's bevel is off, you won't be able to read its actual bevel, but that doesn't really matter. Here's what I find kind of funny, though; how does a chisel whose edge "doesn't hold up worth beans" become your favorite chisel!?