Like any other snake, look for one that's alert, no lumps/kinks when you pick it up, nice healthy weight to it, etc. Lively tongue flicking is good. Check for any scars, mites, ticks, make sure it's got a clean vent..etc. Gopher snakes are mostly diurnal, so they should be somewhat active during the daytime. You may get one that's going to puff up and hiss or shake it's tail at you. This is a common behavior of a gopher snake that's feeling threatened. (Personally, I think it's one of the great things about them).