You would want that point to be all vapor leaving the last run of the coil which should be close to temperature of the air passing into it. If you measure at the entry point, that would give you a better indication. The coil generally would frost up at the expansion valve and progress along the coil as time goes on. If you are reading 32 degrees or less at the outlet of the coil returning to the compressor, the coil has already froze up. Different refrigerants flash off at different temperatures and pressures. For example, at 0 psig (15 psia) R12 flashes at -21, R22 flashes at -41, R502 -49. So one adds the proper amount of refrigerant to bring the pressures up to operating temperatures. So IN GENERAL, an AC unit using r22 would require enough pressure so it would flash off in the coil at about +40 and there is a diminishing amount of saturated liquid throughout the coil until it all is evaporated at the last run of the coil and no more heat is picked up as it leaves the coil, where the work is supposed to be done.
I tried not to get too technical so I hope you get an idea of what is supposed to happen.
As the coil freezes, the refrigerant has to travel a bit further to pick up heat. This causes the coil to progressively ice up. But with the proper charge, the refrigerant never gets to freeze the coil on an A/C system. Different story on a refrigerator and freezer/ice maker.