www.harbourfreight.com (probably a store near you)
has 134a gauges with three hoses starting at $39.95. Get a long digital thermometer and keep it stuck in the middle vent. Look for 41-48 degree vent temps at 80-85 degree Min weather.
The gauges-red goes to high side (going from compressor to condenser), blue goes to low side. (dryer can to firewall).