Several years ago, a new loaner that I had lit a check engine light - I was told that the gas cap was probably loose.

It was - I was told that the light had to be reset at the dealer - AND it was a big nuisance.

Later models have an extra gas cap warning light - but, if you don't fix the problem, it will throw a code.

A Stant replacement cap should be reasonable.

Try the place where you buy gas - they may have a collection.
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'96 855R,'64 PV544 driver, '67 P1800 basket case, '72 Yamaha Rd400, '68 Honda 350-4, '12 XC70, the first 5 are mine, heh, heh, 525,000 miles put on 10 bricks James A Sousa

There is no test that I know of that can check a gas cap for a seal failure. Many states that check emissions have government testing stations, I assume this does not include your state.

If there is an online EPA site for your state, I suggest looking at it and see if there is a complaint section. You are correct, a new gas cap from a big box store is around $10. Normally, if the gas cap leaks, you will get an error code that can only be resolved by a smoke test or throwing parts at the car until it is resolved.