The most accurate way to test an AAV is by using a manometer. However this may not be readily available.
Another way is performing a field test in a cup of water, Place the air admittance valve (AAV) in a cup as shown here, on page 4. This creates a positive pressure, which seals the membrane and allows the valve to float. If the AAV allows air to pass through, then the membrane may have compromised, causing the AAV to sink.