There are recharging kits that come with THERMOMETERS that strap onto the AC pipes.
This allows you to see the temperature of the refrigerant
before and after it passes through the evaporator.
This will be your best indication of being undercharged,
other than bying a set of AC Gauges which are $$$$.

Pete...

98.5 SE Sport Duratec ATX (P66L)
Black Clearcoat on Greystone
DOB 11/21/1997
DOP 12/25/1997
PIAA 510 Super White Driving Lights
SVT CAT-BACK EXHAUST (No Resonator)
DiabloSport Chip (LPQ4) - Engineered To Raise Hell
Too many other mods to list...
Sounds sweet, runs quick...
Crazy Horse Dyno @ 121.4 WHP/129.6 TQ