If the refrigerant is low, the pressure at rest should be about 125psi at rest. You'll be able to see that with the high side gauge, and the low side should max out (they usually max at 100psi). That should be enough pressure to get the compressor to start, and then the low side pressure will drop rapidly until it kicks out because the pressure drops to low, in the 10-20psi range. If the sight glass is on the high side, I believe you are supposed to see solid liquid, no bubbles.
If the gauge is reading lower than according to this chart: Refrigerant Pressure - Temperature Chart
for R-134a, than there is likely nothing in the system, or so little that it'* only pressurized gas and no liquid. If it has no pressure at rest, than there is the possibility outside air has now contaminated the system, in which case you should track down the issue and resolve it, replace the drier, have a vacuum pulled on the system for 30 min to pull all of the moisture out, and then properly charged.
You are correct, pressures should be observed when the compressor is running, but you can see enough without it running as outlined above to try and figure out why it is not running. Causing a compressor to engage with no safety measures is a great way to cause more damage.