without a gauge set looking at both the high & low side of the system while it's running you'll only be guessing & taking shots in the dark. if the freon flow is restricted due to a plugged filter/drier or even a occluded capillary tube it'll show quickly with a sky-high pressure on the high side of the system, & a low side running towards a negative scale. it's also possible to undercharge a slider system when your tech didn't weigh-in the full charge amount & only eyeballed it just using his gauges based on observed pressure.