Use the Get Streams endpoint https://dev.twitch.tv/docs/api/reference#get-streams
You don’t need an access token, a client id will work fine, as shown in the example.
Just make a request to that endpoint, with your client id as the Client-ID header, and you can specify one or more channels using the user_id or user_login querystring params depending on if you want to use their username or their Twitch ID.
If the channel is live, you’ll get a stream object back in the results, if their offline you wont.