You can also just short out the low pressure sensor on the receiver/dryer and see if the compressor kicks on. Doesn't tell you if its low on refrigerant or if the sensor is bad. But its easier than using gages unless you happen to already own them.

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'84 300CD Turbo 132k (Anthracite Grey) - WVO - My daily driver - Recently named coo-coo-coupe by my daughter.
'84 300D Turbo 240k (Anthracite Grey) - Garage Queen
'83 300D Turbo 220k (Orient Red) - WVO - Wifes daily driver

I'm not a certified mechanic, but I did stay at a HolidayInn Express last night.