Fuel tank/lines 71 c-10 questions

[ericfallon]

This most likely is a silly (read stupid) question, So, don't shoot the newbie.

My 71 has 2 lines going to the fuel pump on the engine. from the tank.
I'm "assuming" one is a vent/return and one is the feed.
How does one tell what line from the tank is the feed. and what port on the fuel pump is the inlet for the fuel.
I ask as the truck will start if I pour fuel down the carb, but doesn't seem to be getting anything from the tank. (tank has 10 gallons in it) . The rubber lines under cab were removed from the tank sender metal lines to drain the old gas, and I might have put the wrong hose on the wrong line, and would like to double check this before I "assume" the fuel pump took a dump. I have to replace the rubber line/hose as they are both cracked,aged, It looks like one metal line from tank sender is larger o.d. but. not 100% sure. was under there with a flash light with dieing batteries.. (murphy's law)
Thanks.
Hate to replace the fuel pump only to find it wasn't the problem.