With cameras so common now days, I would think a couple of "NOTICE: Hidden Cameras" signs would be believable, and would do the trick. If not, get some game cameras.
I think a single sign at the entrance reading simply "Buchshot" may work in my area. But since the local rednecks are more scared of moccasins than buckshot, maybe "WARNING! Moccasin Infestation" would work better.
