To check if a user is subscribed to a channel, you can use the Get Broadcaster Subscriptions endpoint https://dev.twitch.tv/docs/api/reference#get-broadcaster-subscriptions
You will need a valid token from the broadcaster for that as it’s private information, and if you need to check users subscription status frequently then rather than individually checking it may be worthwhile to create a cache of subscribers, and the use Webhooks to update the cache to changes (you’d still need a broadcasters OAuth token with appropriate scopes, and it needs to be valid when creating/renewing the Webhook subscription).