snail bait poisoning signs

With Spring here, there are gardens going in and seedling popping up.
If the neighbours use snail bait on their seedlings and the cats
happen to eat some - yes I know it has stuff in it to deter them, this
is worst case scenario - what are some signs to look for? and what
effects would it have on a cat if the signs were missed, say for
example if the cat felt sick and didn't come home for a day?