Great question — answering as the UphillTech team:
You check the pressure with a pressure gauge which should be attached (permanently) to your system near the pressure tank. If you need a quick and dirty check, you can get a pressure gauge attached to a garden hose fitting and attach it to a hose fitting, but the standard pressure tank Tee has a port for the pressure switch and a port for the pressure gauge, right there, and one should be there at all times. There is no single "correct pressure" - there are some standard or typical pressure ranges for well-pump systems, and since the usual non-adjutable differential is 20 PSI, those are 20-40, 30-50, 40-60 and 60-80 (no reason you could not use 50-70 or 35-55, just not typical enough to see...
Full answer by Ecnerwal: https://diy.stackexchange.com/a/113458 (CC BY-SA 3.0).
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