>>When does a chisel become a slick?<<

I don't know if there's an official answer, but typically a heavier cross section at the business end than these chisels (which are about 1/8" thick at the end), a socket, some back relief - as in often not perfectly flat on the back - and a socket vs. a tang type like this. 

I'm sure there are modern versions of slicks that have a ferrule and aren't a socket type, though - now that most stuff is spun on the lathe to make the bolster near the tang. One of the few hand tools I've never owned or made.. western slicks.