This is a test of commonsense. Complete the next sentence:

How to put freon in an ac unit 
Check for vents blowing warm or room temperature air. 
This is a tell-tale sign that your unit needs a refill of refrigerant. However, a broken thermostat may cause the same problem.
If it's a refrigerant issue, the air temperature will steadily grow warmer over time because your unit is slowly losing refrigerant. If a broken thermostat or other issue is causing the warm air, the temperature of the air will probably spike or fluctuate.