Easy enough to do, although if you suspect a leak, might be worth going down the vacuum road instead, to save throwing lots of fuel into your bilge.....
1) Seal exit hose (can do this with an old connector for your engine, add about 3-6" of hose, then fold over & lash / squeeze with mole grips. This will preventfalse readings form the bit that's meant to be open. If you ghave a separate vent, you'll need to do something similar for it, unless it;s one of these clever ones where the vent is integral to the cap.
2) buy a replacement filler cap. Drill a hole, and fit a car valve, epoxy it in.
3) fit cap & blanksing device(s), find a car foot pump & slowly increase pressure, you wonlt need much. (a couple of PSi wil lbe more than enough)
4) this is where you have 2 options. Either watch the gauge on the pump, if it falls you need to search for the leak. or you'll hear air hissing or fuel spraying. that will make finsding the leak obvious.
And before you all go on about volumes of tanks vs foot pumps, I have a twistlock bung adapted in the same way to find leaks in dinghy bouyancy tanks. You really don't need a lot of extra air to get a pressure finding leak. There is anargument for doing this with an empty tank for the petrol spill reasons above.
A possibly simpler way would be to top it up right to the cap (or another easily identifiable feature on your filler pipe), leave it a few days & see if it has dropped.
There is the other thought that my rib spends most of it's time with my favourite ribbing aftershave smell- "Eau d'essense"
.... only after a 2 year engine rebuild with no fuel did the "aura" wear off.