how do I determine the low pressure side of my 1991 chevy caprice a/c 305?

there is normally a schroeder valve on the expansion tank after the expansion valve....do you not see this thing?....think it should be 17 psi (educated guess)~ if you're in for service would recommending replacing the filter/dryer as this goodie removes any moisture laden in the refrigerant too~

....yes, after the filter/dryer, you'll find a schroeder valve on the low side~

Some have an expansion valve instead of an orifice tube. You won't get a true reading this time of year due to no humidity. Low side should stay 45-55lbs in normal hot summer conditions