I'm with the blue tape and sharpie idea... I shoot stills and video commercially, so all my bags have those items already, I use 'em a lot for personal film shooting.
I use blue tape for battery packs and memory cards too - if the terminals are covered with blue tape or the case is closed, it's unused - no tape and it's been used. Works fine for me on hectic set days. I go through some rechargeable AAs, but I have those plastic 4-pack cases which get the tape as well. I tend to fold an end into a "tab" when I use it to pull it off easily. Sometimes stick a little square sticky-to-sticky side for battery packs, so the sticky doesn't get on the terminals. I've found even on 105° Texas days the tape doesn't leave residue though.
Maybe for another thread, but... camera bag-must haves: blue tape, sharpie, pencil, reading glasses (hands-free magnifiers for cheap), LED flashlights. And I always have a small tool pouch.