The /streams endpoint can be used for that if you specify the channel parameter, for example to check whether channel1 or channel2 are online: https://api.twitch.tv/kraken/streams?offset=0&limit=100&channel=channel1,channel2
You’ll get an array with stream objects for the channels you specified that are currently live, so you’ll have to compare what you requested to what you get if you also want to know which ones are offline.