The best way to know if it is full is seeing how much is in the system. Any shop that does A/C freon checks has a machine that can pull out what you have, and weigh it. Whether or not it's a paid service depends on the shop(we do checks at our shop for free)
A full system in a Blazer has .79kg, or about 1lb 12oz.

 

Good info, I'll have to look around for one of those shops then.

 

Something like this work to check the vent temperature?

Instant Read Digital Thermometer

 

When the A/C system is on, low side pressure should NEVER reach 50psi. If the cycling switch is operating properly, low side pressure should cycle between 22psi and 42psi. The exception is if ambient temperature or relative humidity is high. Even if/when that occurs, the compressor clutch might remain engaged but low side pressure must still remain between 22psi and 42psi.

The business about 55psi being "full" is nonsence. R12 and R134a both have similar characteristics: refrigerant temperature and refrigerant pressure are almost equal from zero degrees Farenheit to around 150F. As mentioned, always monitor high side pressure too. If you don't understand A/C, don't risk physical injury to save a few bucks, take it to a professional. Automotive technicians are still cheaper than MD's

 

Yep, just like I said... dont go by feel but go by feel! LOL!

Feeling air temp is very misleading, but feeling the suction line wont be. Chances are if its the right temp it will be cold enough to be sweating.

Thermometers are cheap. I keep one in each of my vehicles just because. less than 5 bucks or so.

I also agree with Capt. Hook. Alot of people are playing with fire by bypassing stuff and not fully understanding what they are doing. Be careful!