The cars suppose to be running, AC on and fan on full blast. If the AC Compressor wont kick in then you will get the red zone. The quickest way to see if the compressor is "ok" would be to jump the low pressure sensor with a paper clip. If you do so and the clutch engage then your compressor is most likely working order and there is no pressure in the system (no refrigerant) You can also get an AC Gauge set if you have one and test it that way. (many guides on YouTube)
* the low pressure sensor is a protection for the compressor so do not leave it jumped, just use to test the compressor engages.
There are other ways to test the compressor. You can use the FORScan and start a Key on Engine OFF test (KOEO) and that round the clutch as a part of the test. (YouTube video there too)
If the test works and the compressor engages then you have an issue with pressure and you really need a gauge set to diagnose. (around $50 at HF or cheaper second hand)
