Under the hood is usually a green cap. I have see them down low near the frame too.

Follow the lines to and from it looking for obvious problems. Those lines should go back to the tank and charcoal canister. I have had people crack the canister's with rocks or ??? so I JB Weld (Plastic Weld) the cracks and good to go. A quick look may save the cost of a smoke machine or smoke test.