Batteries, I always have three tons at home.
Whether they are full is another question!
If you're like me, don't hire a private investigator.
Here's a cheaper trick. Just bounce the stack to see if it's still good:
Contents
1. Stand 15cm above a table and drop them:
2. If they bounce once and fall straight down, they are full.
3. If they bounce again and again, they are empty or close to empty.
There you go, you now know how to recognize an empty stack from a full one :-)
Easy, practical and effective.
In addition, it saves money because you no longer throw away full batteries! And if they're really empty, you know you need to buy new ones.