Many times you can smell a strong odor of chlorine when you get close to the pool. Or if you're in the pool sometimes your eyes will start to get irritated, however this may be from the PH levels being out of balance.
There are ways to tell if the chlorine level are too high such as to the lightening of the pool liner(assuming it's an above ground type), an asthmatic person may start to wheeze and eyes will run. If your pet won't stay near it,the that is another sign that's it's too high. You really need to test it.