How do we know which direction we should face when we pray?

There are ways of calculating this - you need to know your current location, the location of the qiblah and also some way to determine direction.

Most android phones have built in GPS (and most/others can also use other means to determine a good approximate of the location), and a built in compass. This should be enough for some app to combine the data and let you know which direction to face.

I am not the first with that realisation - there are a few apps on the Angroid Marketplace which say they can locate the qiblah/direction to face when praying. I tested the free options.