to test if the refrigerant is low, have someone who knows what they are doing jump the low pressure switch for a second and see if that kicks the compressor on. the low pressure switch prevents the compressor from engaging if the refrigerant pressure is too low (because the compressor can be damaged if it runs in that condition).

__________________
2005 Mercedes C230k sport sedan, 6 speed
1987 Porsche 924S - 968-engined track car