Check the temperature differential. Charge the refrigerant if needed.
A solid maintenance will generally include some way of measuring refrigerant levels. However, if there’s no indication that there are leaks, then why risk removing refrigerant, or adding oils. If the system appears to be (visually and sound-wise) operating fine, the suction line is cold, and the temperature differential (between the air duct and return air) is at spec, then it may be overkill to attach the gauge. However, if any of those seem out of whack, a more thorough examination will be required. What should be done at a minimum is a complete cleaning of evaporator and condenser coils. It’s fast, simple and shows the customer that you care about the system.