If you had a specific list of channels to check, you could make a GET request to this endpoint with a comma-separated list of channels to check if they’re online or not. Reference.
https://api.twitch.tv/kraken/streams/?channel=a,b,c
Make a GET request to this endpoint to get the general summary of channels on Twitch. You can specify a game in the query string to limit to just that. Reference.
https://api.twitch.tv/kraken/streams/summary
https://api.twitch.tv/kraken/streams/summary?game=Counter-Strike:%20Global%20Offensive