The Subscriptions API returns paid subscribers
The Follows API returns followers
This is a URL to the paid subscriptions API
You called the wrong API, you called “Check user Subscription” - Reference | Twitch Developers
When you should of called “Get Broadcaster Subscriptions” - Reference | Twitch Developers
Get Broadcaster Subscriptions uses a token with channel:read:subscriptions where the token user ID = the broadcaster_id
Check User Subscription users a token with user:read:subscriptions where the token user ID = the user_id
So you had the right params, you just called the wrong API.
So
https://api.twitch.tv/helix/subscriptions?broadcaster_id=123&user_id=321
Will look up if 321 is subbed to 123 when using 123s token
And
https://api.twitch.tv/helix/subscriptions/user?broadcaster_id=123&user_id=321
Will look up if 123 is subbed to 321 when using 321s token