A simple solution for indicating whether or not a rechargeable battery is fully charged or spent is to wrap a rubber band around the fully charged ones.

When the battery is picked up for use, the rubber band would naturally have to be removed. So if you find a battery in your equipment case that is not "sealed" with a rubber band, odds are that it has been used.

Some folks use vecro stickers (such as double sided red/green) to indicate charged or spent status, but sometimes a crew member will forget to flip over or reposition the velcro indicator.

Also, one might be dealing with rental equipment that is not velcro enabled.

Rubber bands are plentiful, non-destructive to property, and fool-proof.