You can request no scope at all &scope= and use that returned token along with the root call (https://api.twitch.tv/kraken) to determine which user that token belongs to.
However, keep in mind that relying on only the token as the validating credential with implicit grant flow is inherently insecure since tokens generated for other applications can be injected into yours to authenticate the user.
The Authorization Code Flow and relying on the code parameter as your validating credential provides better security since an OAuth token can only be generated by knowing your applications client secret.