There's no plug to check. Instead, to determine if there might be a problem with the dryer (which would usually manifest as a lack of cooling or poor performance):
* Visual Inspection (difficult and potentially risky): You could try to locate the dryer itself (usually near the compressor). Look for leaks, damage, or anything obviously amiss. Warning: AC systems contain refrigerant under pressure which can be dangerous if handled improperly. Do not attempt to service the system yourself unless you are a qualified technician.
* Professional Diagnosis: The best way to tell if there's a problem with the AC dryer is to have a qualified automotive AC technician inspect the system. They have the tools and knowledge to properly diagnose leaks, check refrigerant levels, and determine if the dryer needs replacement. They can also check for other potential issues that could be causing your AC to malfunction.
In short, there's no "plug" to check on the AC dryer. If your AC isn't working, you need a professional diagnosis.
