Stumpfarmer, It's not foolproof but they do seem to sense what's ok and not ok. Can't ever recommend that in my position, as there's no scientific proof they have any way of detecting that. But personally I think if they're dumb enough to eat something poisonous I didn't want those genetics in my flock anyway.
Edit: That'd be a fun topic, what is the dumbest thing you've ever had a chicken do? Regardless of surviving it or not. Mine is I once had a full grown adult OEGB drown itself in a baby chick waterer...he was in a 2'x4' conditioning cage before a show, put him in there one night, next morning he was face down in maybe 1/2" of water...