How do I tell if an add-on was snuck into my car contract?

Read the itemized buyer's order and retail installment contract line by line before signing, and compare them to the price you agreed to. Look for service contracts, GAP, etch, protection packages or other products you did not ask for, and check that the amount financed matches your expectations. If something is there that you did not agree to, ask for it to be removed before you sign. The FTC and CFPB both publish guidance on spotting hidden add-ons.