How do you know when the battery is low or flat

Question

My watch lost about 2hours i have reset it and it seems to be keeping good time the last 20 min.I just wanted to know if the battery drops in voltage ,does the time slow down or just stop with the other funtions as well.
Then there must be another explanation for the time, to advanced by 2hrs .In setting the time you have o hold down the buttons so .