While I don’t know for the progamming language Swift, you would check if there is any data/objects in the 'stream' token at the link: “https://api.twitch.tv/kraken/streams/CHANNELNAME?client_id=CLIENTID”
Two examples below, notice how the ‘stream’ token is null in the offline user but it contains data for the user who is streaming.
Example of user who is not live:
{"stream":null,"_links":{"self":"https://api.twitch.tv/kraken/streams/larklen","channel":"https://api.twitch.tv/kraken/channels/larklen"}}
Example of user who is live:
{"stream":{"_id":23808949568,"game":"Hearthstone: Heroes of Warcraft","viewers":3756,"created_at":"2016-12-01T16:54:04Z","video_height":720,"average_fps":60,"delay":0,"is_playlist":false,"....and so on, much more data is shown in real-time.
Hopefully this helps! (if you were looking for a Swift-specific answer on how to make an api call, then sorry! Maybe this Stackoverflow link will help?)