If your AC unit has frozen up, there are a few ways to tell. First, check the exterior unit to see if there is any ice or frost build-up. If so, then its likely that your AC unit has frozen. Second, check to see if the air coming from the vents is cool or warm. If its warm, then this is another indication that your AC may have frozen up. Finally, check your electricity bill for any spikes in usage; a frozen AC unit can cause your electricity bill to spike as well. If you notice any of these signs, then its best to call a professional AC technician to come and take a look at your unit.