Since sealed units dont have a service port for gauges which makes checking to see if it is low on refrigerant not a quick as with other HVAC systems.
Installing service ports can be done and is not a difficult job but some basic things can be done first that will give you a general idea.
Using a Amp meter and a IR temperature gun we can quickly tell if there is no refrigerant flowing inside a sealed system.
Amp Meter Method
Most small compressors installed in window units or similar will run around 2 Amps. Of course check the face-plate to be sure what amps it uses.
If the compressor is not working and pumping refrigerant in the system than amps will be much lower than the standard amps.
For example a compressor that regularly uses 2 Amps will be using 1 Amp or lower.
Of course the compressor doesn’t always run and shuts off and on, so be sure the temperature is set lower for the unit to kick on.
If the amps are low it does not automatically make the compressor bad but does point the the fact that refrigerant is not flowing in the system for what ever reason.
For example a unit could have a leak.
Using a Infrared Temperature Gun
A IR temp gun (or any Thermometer) can be pointed at the condenser to see the heat it is giving out.
Most condensers will run around 100 degrees so if it is low than you will know there is a problem.
Of course be sure the condensing fan motor is running pushing air through the coils and dissipating the hot air.
The above method are just some simple things to quickly check when you approach a sealed HVAC system and want to see if there is refrigerant flowing.
There is no doubt many other methods that can be used to troubleshoot a unit.
Somethings will even be obvious such as the noise the compressor is making on the window unit.
If in doubt installing service ports is a easy job with the right equipment and can be done easily.