For AC work there is something called a Manifold Gage Set, Two gauges, Two valves and 3 hoses
Two of the hoses connect to the HIGH and LOW pressure ports on your A/C system..THe fittings will NOT connect to the wrong hose.
The 3rd hose to a tank of Refrigerant, and then the two valves decide if you pump high pressure refrigerant into the tank or if the tank feeds into the low pressure side.
These are the gauges.
Link to Amazon catalog pageYou can also get a single (Low side) gauge it is much less expensive and nearly as informative as the full set in most cases (But not all)