The PO of my 72 installed new stainless steel fuel lines and the third line is rubber fuel line. How do I tell which one is from the tank to the pump and which one is the retun to the tank? I assume the rubber hose is vapor recovery or something like that? Do all fuel pumps reside in the engine compartment?

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If you have a factory fuel pump, and it sounds like you do since you reference 3 hoses, then the diagram above may help. I do remember that the "D" port on the fuel pump is "Druck" (maybe sic) which is German for "pressure"; can't recall what S and R stand for but the R is a return to the fuel tank... Note that the fuel filter in the "below car" diagram is from the tank, before the fuel pump so that "S"ucks gas from the gas tank; D is pressure gas feeding the fuel rail; R is a return to the tank loop...

In MY 75, they moved the fuel pump to the front compartment

The return line to the tank is usually smaller than the feed line.

any idea how to verify if the po installed the fuel lines according to the above diagram?

I would think only by verifying the sizes of the lines. It may be that you have the CFR SS lines; in his set they are 2 sizes so hopefully the PO installed them correctly. Get under the car and look at the lines. If you're questioning the rubber hose connections between the feed lines and the fuel rail being correct, I don't think the car will run if the fuel lines are reversed; easy mistake to make and one that has been discussed on the World before...