- In GSAK, find another geocache hidden by that same Geocacher. Right-click on the record and select Edit. You will see the Owner ID populated midway down the screen.
- If there isn’t another geocache in your database hidden by that same user, go to geocaching.com. Find their Public Profile and click on See the Forum Posts for This User link. You will navigate to the Forum. Now, look at the URL at the top of your browser. The last digits in the URL address is the cacher’s User ID.
NOTE: I'm not affiliated with GSAK - just a very enthusiastic user.