As I have learned from this forum and my A/C service person, most systems (usually furnaces) are rated for a maximum external static pressure. However, to really get the measurement, holes must be drilled into the cabinet without damaging the equipment. I think it would be a great idea if manufacturers would put access holes in the cabinets at the time they are built, put removable plugs in, and put information in the service manuals about where the holes are located. This way, it would be easy for a technician to get a reading. There could even be an access hole put in the evaporator coil cabinet on the supply air side of the coil so someone could read the pressure drop across the coil. Maybe it has been tried and was not feasible.