Use the /users/:user/subscriptions/:channel endpoint with the channel name and the access token. You can see example data and status codes on the page I linked. Use the status codes to check if a user is subscribed/not subscribed.
Use the /users/:user/subscriptions/:channel endpoint with the channel name and the access token. You can see example data and status codes on the page I linked. Use the status codes to check if a user is subscribed/not subscribed.