If you can locate someone with a professional scan tool, get into the datastream mode and monitor the Fuel Tank Pressure. Remove the fuel cap and see if the tank pressure drops.
While the scan tool is connected, go into the component parameters and check the EVAP components for reported faults.
Or, if you are comfortable working on your car and playing with the electronics and components, you can check the EVAP purge solenoid for proper operation. Also, you could replace the fuel tank filler tube and check valve. You'd also want to inspect the canister vent hose and canister vent solenoid for correct operation.
-Rod