The new Vision gives onscreen battery info but us ancient (last year!) V1.x Phantoms rely on timers, FPV or other tx hacks. Hobby King sells a $26 voltage alarm which looks cheap when it comes out of the box but weighs nothing and works well. Good backup info to your timers. Anyway, I couldn't see a decent review of it on Youtube so I made my own:
http://youtu.be/YhgEv7In7Cs
http://youtu.be/YhgEv7In7Cs