If the gas cap is tight and light still on, buy a OBD II diagnostic hand held. It's about $100 and plug it into the diagnostic port under the driver dash. It will read the code and tell you what's wrong. Car parts stores like Kragen, Pep Boys, Auto zone all sell these.

Many small private shops will check the code for you for free or a small fee since it's a 5 min job. Dealer charges 1 hr of labor which is usually over $100.