When the battery gets low you will notice a little distortion on the output signal. It's like somebody mixed a little bit of overdrive behind your clean signal. The good news is that the battery lasts a really long time. If I remember right, the voltage is will have dropped to below 5.5 volts before the distortion is noticeable.
And as if by magic, my battery gave up the ghost last night. I think I put it in last June. That time is shared with the Silhouette Special so it's not quite the seven months it seems, but I also frequently leave it plugged in over night. The average home player should gets six months with no problems, in my humble opinion.
So, low battery symptoms are: no volume drop, a little distortion that you'll notice on pristine clean sounds, but when I stepped on my distortion pedal, I got radio buried in the background noise, which was my cue to change!
Battery voltage measured 3.4V, which is pretty incredible. Most consumer electronics will give up the ghost long before that.