when you park it for the night or whatever after driving it, look around and underneath of the car in the morning or after a couple hours too see if you can spot anything...also, look under your hood and rev your car a couple times to see if you can see anything leaking

 

Do not top off you tank completly...because the exaust gas return line that comes from the smog canister will get full of fuel. so thats why you smell it coming thru the air vents.