As said start engine, turn AC on and compressor should start to cycle on and off, it should do this even with AC off but turn AC on just to be sure
If its not coming on then either refrigerant has all leaked out, or compressor clutch is bad(unlikely)
Here's the problem, you can end up spending money on refrigerant you don't need and if there is a leak you lose that money, or if a part is bad and needs to be replaced then you also lose that money because system needs to be opened and you lose refrigerant
I would spend the money for an AC guy to test the system and tell you whats wrong, then YOU can replace the parts you need to instead of guessing at what needs to be replaced
AC systems are not complicate in how they work, the compressor COMPRESSES the refrigerant, I know duh, this makes the refrigerant very very HOT and under high pressure, which is "stored energy".
Then there is an Expansion Valve, when this HOT refrigerant is allowed to UNcompress/expand, it get VERY COLD as the "stored energy" is released
And you have AC
Now the compressor is either ON or OFF, there is no TEMP CONTROL for an AC system, the temp control in the cab moves the Blend Door inside the cab, that sends air from the fan either thru the Heater Core(HOT) or around(bypass) the heater core(COLD), or anywhere in between
AC system diagram here:
https://www.2carpros.com/images/articles/original/ac-system-645.jpg
Home AC, refrigerators, freezers, ALL use the same system, compress the gas to heat it and then un-compress the gas to cool it
Repeat, lol
There are certain aspects like the "high side" pressure, and "low side" pressure that have to be in a certain range for the system to work, and not Burn Out the compressor or Blow Out a connection
An AC guy will have the high pressure gauges to test system and to see if system can even hold pressure, no leaks
You can learn all this and rent the gauges needed to test it, loads of videos on how to DYI automotive AC systems
Thats up to you
And as you may have surmised by reading the above, its NOT working above 30MPH, if AC Compressor is not cycling at idle, lol.