Use Knowledge checks to determine the creature type and learn what normal AC for creatures of that type should be. Check their Natural Armor bonus.
Then compare to armor that they are wearing vs. what's listed in the MM - if any, and check the gear's quality. To be magical it has to be of masterwork quality.
If you have Detect Magic running, you'll see what gear, if any is magical.
This will at least give you a minimum AC to base your decisions on.
Not a great idea... but when you really, really, really have to know exactly what the AC is for a foe... Then you could have the party wizard cast Probe Thoughts. It's overkill, but it'll get you the information you want if they fail their save. Plus other goodies, if you have lots of time.