How can I determine the UserID associated with an eBay token?

Note that an eBay user can change their UserID, and this will not affect the validity of any authorization tokens associated with that user. If you need to find the UserID associated with a given token, you can make a call to GetUser (DetailLevel=ReturnSummary) with only a token.

The UserID field in the response will be the UserID associated with the token used to make the call.

Identity API:https://developer.ebay.com/api-docs/commerce/identity/resources/user/methods/getUser

Trading API: https://developer.ebay.com/devzone/xml/docs/reference/ebay/GetUser.html