How do I find out which product key was used to activate Windows?

For Windows 7, 8, 10 you can use the slmgr command line program to reveal a portion of the key to help narrow it down.

1. Open Start Menu, then type:

   cmd

2. Right click CMD application in the search listing

3. Then click "Run as administrator"

4. At the command prompt, type the following and press enter:

   slmgr -dli

5. Wait for a little window to appear and then look at the value to the right of "Partial Product Key:"

   

6. Compare with and match to your windows product keys at hand. Note that this reveals the right most part of the ................-3V66T

+1 on Magical Jelly Bean Keyfinder. I've used it many times to help friends recover their Windows product keys. Link