How can I check if a user is subscribed?

Hi i read from the docs

broadcaster_id -> User ID of the broadcaster. Must match the User ID in the Bearer token.

but in order to obtain the User OAuth Token, we usually must create a link to https://id.twitch.tv/oauth2/authorize with parameters client_id, redirect_uri, response_type, state.

After authorize it, we receive a json with access_token (that we use as Bearer token).

So, please me more regarding this broadcaster_id to insert into Bearer Token.

Thank you in advance