How do I know if I'm experiencing double vision or not?

You will know. If you are seeing two of every object, you are having double vision. If so, you should be evaluated by your eye doctor or general doctor.
Two ways. True double vision (binocular diplopia) occurs when you see two distinct objects, because the eyes are misaligned. Try closing one eye and the the opposite eye. In these situations you should see only one object. With both eyes open you should see two distinct objects.