Low refrig would be indicated by foam/bubbles.full charge would be clear ..
That is if the unit was not converted to R134...many were. R134 will show some bubbles even when fully charged,,that's why you would need gauges and sw pressure cut-in spec to make that sw diagnosis if R134.

Usually you can get the high pressure sw to trigger if you try the a/c at a high idle on a hot day. Can even block the radiator a little to getthngs up there. Just use your head.

On you coment of a manual toggle in the cabin..this is an excellent idea used by many on the older Benzs..you put a toggle in the cabin wired across the a/c pressure sw. that it gives you control of low fan anytime you see the temps creeping up..this is excellent in traffic or towing, etc...the niceity of it is that it does not interfer with the stock system, but lets you over-ride the sensors to get a jump on it...best of both worlds..I have used one on all my Benzs over many years and they are just a nice saftey feature to have when that one time comes into being..like stuck in traffic at the Beach, ya know??????
The newer units have fan default if a sensor goes, so they do not need it, but the older units don't , so you just add this and you are in control..