Yeah, whenever my bunnies eat something they're not supposed to I do what Pipp said. Also, you might want to try to listen to his stomach for gurgles--try to hold him up to your ear like a telephone. This can result in large scratches to the face, however, so be careful. It's easiest with a small, non-squirmy bun. If you hear gurgles, I'd give some Gas-X. Another thing to look for is the neurological problems of poisoning--I do this by checking pupil dilation (of course, there are other more obvious signs, but if they're not present I check dilation to be sure that everything's ok). Shine a flashlight in his eye and check that the pupil will shrink normally, and then when you take it away, it should dilate normally again.