The same way that you do with the a/c compressor running. You connect the hoses to their respective fittings and read the pressures. In the case of static pressure you only need to connect one hose.

If you have about 80 psi then you have enough pressure that the high pressure switch should be activated. If you have something less than about 75, then you have a low charge.

Seeing if there is a charge is always the first step in troubleshooting a system in which the clutch will not engage. If you don't take this step you can easily find yourself "chasing your tail."

Good luck,