My 1999 Mazda Miata just had its Check Engine Light come on. I ran the codes on it and it is a PO455. A quick check online tells me it is something leaking in the EVAP system. Many say to change the gas cap. Others said to change all of the vacuum lines. Any way to know better what the issue is without doing all of this presumed fixes? Car runs fine so far...no blinking CEL yet.

 

Could be faulty purge valve solenoid.

 

Rayzcar: you go from cheap to costly. Inspecting, cleaning and reusing is cheap.
The cheapest is inspecting and properly tightening the gas cap. You want a clean gasket and you want to hear 3 clicks when you tighten the gas cap.
Next is inspecting the vacuum hoses for cracks and such, also checking the solenoid and the wiring.
So far you haven't spent anything. Replacing the vacuum hoses is next because you can buy the hose cheaply.
After you have done something clear the codes by removing the battery negative cable and then stepping on the brakes. The brake light circuit will drain all voltage from the memory chips.