how can u know if your gas cap needs replacing on a 2002 Mountaineer

The first thing you should do is take this to a local O'Reilly's, Advanced, or Autozone. Have them hook the computer up to your car and view the codes. Assuming the CEL came on because of a gas cap is probably a bad idea.

Assuming that, ptkilbo. Yes I must agree. It does not necessarily mean that is why the CEL is lit. But the OBD won't say "gas cap" it will show something about the EVAP recovery fault, and one way to find out is to buy a new gas cap and see if that solves it.

Exactly, thank you David.