Yes, depending on what data you have from the user you can call the Get Users API.
Got a user token - use token only (no params)
Username - use login param
If you have a access token already you have a ClientID, as you use the ClientID and ClientSecret together to make tokens (usually, unless implicit auth then it’s just ClientID)
Like your other question
Both your ClientID and ClientSecret are obtained from the Developer Dashboard, and manually copy/pasted into the Application config
For completeness the Validation Endpoint (Authentication | Twitch Developers) will return the ClientID for a given token, but if you have a token, then you already have a ClientID.
You may have used someone elses token generator (which is generally not advised), and if you did, you’d need their ClientID (which you can get from the auth URL they use) but you won’t be able to get their ClientSecret, which is why you should get your own ClientID/Secret from the Developer Dashboard Twitch Developers