Boston ba745, if I read your original post correctly then you are looking for a way to tell if your fuel tank is full when all the pipes are behind the cowl. I think the simplest way is as campgems explained and that is to crank the fuel pump handle until fuel comes out of the exhaust then back off 2 or 3 turns of the fuel pump. I do things this way, using a three line setup so with the exception of the fuel dot (which is easily accessable) there is no need to unplug any of the other fuel lines. I also count how many turns of the hande it takes to fill the tank, for instance an 8oz tank tanks 28 full revolutions of the fuel pump handle, so by 25 I start to turn more slowly.

If you are using a two line setup with a fueling valve I guess the same would apply.

Are you using a 2 or 3 line fuel tank?