How to know the battery is enough for fly

i use apm output port to supply power to apm board. is there any way to check the battery power during flying?

Before you ask anymore questions, please read the manual:

but when use the power module what will happen if i connect ESC BEC to apm output port? how should i set jp1 then?
from wiki i saw:"You may “control” servos from the signal pin of A9, A10 and A11 on the left side of the APM but the servos must be powered by a separate ESC meaning you should detach jumper JP1 and use a separate BEC (or the BEC built into your ESCs) to power the APM’s rear power rail and then attach the servo’s red and black wires to the rear power rails’s middle and bottom row respectively."
what that means