Call the users API with just the oAuth token and no ID as a query string argument
For example, from the docs
curl -H 'Authorization: Bearer cfabdegwdoklmawdzdo98xt2fo512y' \
- H 'Client-ID: the_clientid_for_this_token` \
-X GET 'https://api.twitch.tv/helix/users
Will return the user associated with that Bearer token
Or the validate endpoint