97 S-10 a\c problem

My a\c compressor will not come on when I turn on the switch. I have a 97 S-10 that does not have a low pressure switch on the low pressure side of the system.( I have been told that a computer chip reads the high side to control this function) I have checked the pressure in the low pressure side, it is 43psi which is 2 psi short of max pressure. I guess my next step is to check the relays on the fire wall( yes I have checked all the fuses and they are all fine) so I guess my real question is how do you test the relays to find out if they are good or not. Thanx for any help you can supply.