fuel tank sucking jet pump

[Rev Engineer]

Apparently I have a problem with the fuel transfer on my newly acquired 2001 X5. The previous owner said that the gauge was 1/4 tank off. The instrument cluster test shows the left tank having 30-40 liters and the right side only has a few. So, this means that the fuel isn't transferring. The previous owner said he had replaced the fuel filter and I can see non-bmw style hose clamps on the filter. From here it seems like it still be several things: fuel pump is wearing out, fuel pressure regulator is bad, the siphon jet needs to be replaced, or something isn't connected right inside the tanks. Can anyone help how to diagnose this a little further before throwing some parts at it? I took a peek inside the fuel tank on both side and didn't see anything unusual, I also confirmed that the floats work via the instrument cluster test. Perhaps I should pull out the jet pump and check it for a blockage? Can I learn anything by checking fuel pressure or doing a fuel delivery rate test? Anyone know the specification? Or, any other ideas on how to diagnose this problem?