Android : How to know an Android looks whether upwards or downwards?

I'm now creating AR application for Android, but a problem was happened. By my implementation, a sensor show a same pitch even though the Android looks upwards or downwards. Following picture shows what I want to say.

The sensor shows that pitch == -90 if I grab Android horizontally. The problem is that I cannot detect a != b.(The sensor says a == b.) Can anyone knows how to know an Android looks upwards or downwards? The orientation of Android must be portrait.