For $20 you can get the refrigerant with gauge from Autozone or Advance. Instructons are on the can on how to use it. Basically, remove the cap on the refrigerant line near the battery side and plug it in. Note this is only if you suspect that your warm air is due to low refrigerant.
Edit. Nate S 3.0
The cap is to the left of the yellow clip for the hood rod.