. How can i tell which one without replacing both?

Typically a failed VENT VALVE SOLENOID will result in difficulty refueling. A failed PURGE VALVE will result in temporarily flooded engine which results in difficulties starting engine after refueling.. Holding accelerator pedal to floor will help clear flood engine and aid with starting after refueling UNTIL repairs can be completed. All that said - a professional scan tool can perform functional test of the EVAP components to check for proper operations - when used by knowledgeable personnel. As a side note - habitually topping off fuel tank when refueling will dramatically increase the possibility of failed EVAP components!! Good luck!

Come we can check your car for free if you do the work here. We are in 10530 Carmenita Rd. Whittier, 90605. Tell the guys you come for Repair Pall