Your cat's reaction to specific toxins can vary, but there are some signs and symptoms to let you know something is wrong. A few to report immediately to your vet include abdominal pain (your cat's stomach will be sensitive to the touch), a chemical odor on the body, coma, convulsions, diarrhea, excessive drooling, nervous twitching, and vomiting.
If you suspect that your cat has eaten something toxic, call your vet or the ASPCA 24/7 Animal Poison Control Center's (APCC) hotline at 888/426-4435. Stay calm and be prepared to offer as much information as you can, including details on your cat's breed, age, sex, weight, what you think your pet has ingested and when, and the symptoms. There is a fee for using the ASPCA hotline, but it will most likely be a small price to pay in exchange for possibly saving your pet's life.