To find a leak in the fuel tank, fuel lines or charcoal canister a repair shop will use a "smoke machine" that generates an inert gas that locates the source of the small leak, or they may use a vacuum pump to test valves or even a scan tool to close valves and test them. Nothing you can do yourself.
Pat when you say inert gas that is based on the tech using Nitrogen in the test procedure. Most techs do not use Nitrogen in testing with a smoke machine. When using a smoke machine with just shop air you are inviting a fire.