I assume checking it would be no different than any other refrigeration system, as would subcooling, regardless of refrigerant. But what kind of ballpark readings would we expect, what kind of ambient or box temps do we need to consider and other conditions...hot pull down, evaporator coil frozen over etc.?
Is it pretty much like simple conditioning? Like I would expect my superheat to be high if the inside coil is absorbing heat from warmer unfrozen product in a walk in freezer? Lower SH if the evap. coil is obstructed with ice? Higher if the seal around the door is shot?
Go to where the superheat is. At the TXV bulb. Or the evaporator outlet to take your temperature reading. Get your suction pressure as close to the evaporator outlet as possible. Pull out your P-T chart and convert that suction pressure to temperature using the chart, and subtract the difference. That will be your superheat.
If you have no information on the unit, I would generally go with 10-15 SH and 10-20SC. Now if you try and use these numbers during adverse conditions you may find you are adjusting the charge later. I also like to check these when i am closer to normal operating conditions. I wouldn't be too concerned about subcooling on a walk in freezer until my temps were well below 32degrees and closer to normal for ac. Now, this is if I have no other information. Some units run much lower superheats when operating normally. But if you are trying to get a system running and have no clue what the manufacturer wants I go with the 10-15 and 10-20. It will get you fairly close to a proper operating system.