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.
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