Hey Liberator, You can check by pushing a few squirts of fuel in then reverse and pull it out with the same hoses if you get fuel out - that would be your pick-up line. No fuel out then that would be your vent line. This works because your vent is at the top & the pick-up is at the bottom, so with a 1/4 tank, only the pick-up should draw fuel. If your running a three line system, then lean the plane forward[ remember this is only about 1/4 of a tanks worth of fuel] draw on the fuel if you get fuel then yor on the fill line. The premise is that your pick-up line is at the back of the tank[hopefully] the vent is at the top about 1/3 of the way back & your fill line is strait in th middle. Hope this helps.