The best way to determine that air is the problem is to hook up a clear hose in the return system. If air is coming through the return system then it is probably getting into the system on the inlet side somewhere and causing problems with the fuel system.
Air can enter the system two ways.
The first is a bleedback where air is evident after the engine sets inoperative for a period of time and the fuel drains back to the tank and the system has to be reprimed. The lift pump is one of the most common causes of this situation, however any opening in the fuel system can allow this situation. Constant air out the return and constant rough running, missing and white smoke can also be an indication of air in the system. In this situation air is usually entering the system somewhere prior to the pump inlet. If a fuel leak is evident this should be checked and repaired first, however a fuel leak does not necessarily have to be evident where air is entering.