IMHO, your best detector will be your nose..... it's already diagnosed that there's a leak, now all you have to do is find it. The thing is, there's only 3 places it's likely to be:
1) In the engine compartment, most likely where the fuel hose is linked to the injector system. As soon as you notice the smell in the car, you should stop (if still driving), pop the bonnet, and see if you can detect the smell in the engine compartment. If yes; then the leak is here somewhere; further diagnosis can be carried out once the engine has cooled (use a 12v battery to power up the fuel pump, and just go hunting for the leak).
2) In the fuel lines between tank & engine; this will be the most difficult one to spot, but IMHO if you suspect dodgy fuel lines, you should replace them anyway. It's not massively expensive if you do it yourself, just difficult and annoying (depending on what car it is).
3) Around the tank; as you say it only occurs when the system is pressurised, I'd say the tank is probably not what's leaking; but there may be a problem around the return pipe or fuel pumps - which may or may not be in the tank, depends on the car.
My gut feel is that it's a fuel line, but without knowing the car, its age, and how rusty it is (or isn't) underneath, it's just that: gut feel...
