Oil Fittings Plugs

[GMC-DMAX]

Where the factory oil filter mount is, there are two plugs. I believe they are 1/2" plugs. I would like to find out which one has the pressure and which one is the suction. I know if I had a guage say 0-70 psi I could test them , but i don't have a guage. So if anyone, out there know's the answer, of these two plug's, I would appreciate.

Thanks