You can start visually inspecting all the vent lines to the tank. Use wire ties as hose clamps on the hoses going to all the tank vents and the charcoal canister. I use to work at an E.P.A. shop and almost every car failed because the vent hoses were not clamped. These were new high end cars.
The machine used to detect vapor leaks is the same or similar to the one used for emmission testing. Look at your gas cap to see if the seal is compressed or damaged.