The serpentine belt on your Mustang uses a tension pulley designed to automatically keep the belt under the proper tension. Thus, there is no way to measure an actual value to see if the belt is under proper tension. Rather, proper tension can be verified by the condition of the belt. If the belt is frequently squealing, this can indicate it is not taut enough and is slipping on the pulleys. A belt that is too tight will wear faster than normal, showing early signs of fraying and cracking. Serpentine accessory belts are a wear item, and Ford specifies a maximum 150,000 mile interval before replacement. Granted, it is possible for a belt (or tensioner) to wear out prior to this mileage and thus a visual inspection for cracks, rips, or any fraying should be conducted at every oil change.
Changing a belt can be done in a matter of minutes. Noting the belt routing, a ratchet placed onto the tensioner can allow for the tension to be relieved. Removing the belt and reinstalling the belt the way it came off is crucial. Afterwards, relaxing the tensioner allows for the tension to be reinstalled, finishing the installation of the belt.