To check if user subscribes to a channel, you must use the /users/:user/subscriptions/:channel as your method argument (Don’t forger to replace :user and :channel with the actual values.
method: '/users/the_user/subscriptions/your_channel'
Required scope: user_subscriptions
The Twitch API Documentation also describes the responses for this endpoint. A channel object is returned if the user is subscribed to the channel and a 404 Not Found error if user is not subscribed.
Also, I’m not sure if this is the library you’re using, but you should check out the examples given to perform more API requests.