The pumps have multiple sections. Some bolts hold the two sections together and other bolts hold that assembly tight to the block.Those bolts if I remember also squeeze the sections together so Make sure the bolts that don't hold the assembly to the block are tight. Pumps can suffer tears/leaks in the internal diaphragms. If that happens, you may see gas leaking out of the back. There are check valves that can go bad but when they do, you wont see any leaks. You can have an internal leaky diaphram that affects pump pressure but also doesn't leak out the rear.
Note also that the pumps are connected in series so if one is bad, if affects the whole set ( 2 or 3 pumps).
What exactly are you trying to diagnose?
Those pumps wear out and depending on the model year and when if ever the pumps were replaced, they may need replacing. Older lp pumps used a different material for the diaphragms that didn't hold up as long as the newer diaphragms.
On the edge of the pump assembly you should be able to see a small plastic tab that is part of the diaphragm. You may see either a black or clear plastic tab. Now for the kicker; I don't remember 100% what the newer parts are, black or clear but if I had to guess, I would guess that the older ones were black.