Just put 2-3 ounces of fuel in. Then try to draw it out. If it come out, that is the clunk line. If it does not come out, that is the vent/pressure line (based on a 2 line system). Actually, if you accidentally try to fill thru the vent line, in a 2 line system, it will quickly force fuel out the other line even though the tank is far from full.