This checks to see if the current user is subscribed to a thread. You can also get a repository subscription.
Note that subscriptions are only generated if a user is participating in a conversation--for example, they've replied to the thread, were @mentioned, or manually subscribe to a thread.
GET /notifications/threads/:thread_id/subscription
Status: 200 OK
{
"subscribed": true,
"ignored": false,
"reason": null,
"created_at": "2012-10-06T21:34:12Z",
"url": "https://api.github.com/notifications/threads/1/subscription",
"thread_url": "https://api.github.com/notifications/threads/1"
}