how do you know when you have a code po495 problem

I would suspect you have a code P0455 - large evap leak, I don't see a code P0495. If you have a code P0455 stored in PCM of your 2002 Chevrolet Blazer and the code was set due to a loose gas cap the light should go off by itself after driving for a few days. The other option would be to clear the code using a code reader or scan tool.