Each twitter account comes with a unique numeric identifier in addition to your handle. If it didn't you wouldn't be able to login with twitter on some websites when you change your handle.

https://dev.twitter.com/docs/platform-objects/users

EDIT: After rereading your comment you might have been asking if it was possible to retrieve this id with a simple API call and not with OAUTH. It is possible with "users/show" as explained here: https://dev.twitter.com/docs/api/1/get/users/show