If you mean whether a user is subscribed to a product, then you can use this API endpoint to check that: https://market-api.hivehub.dev/rpc/subscriptions?product=eq.peakd-explorer-subscription-and-hive-open-market-introduction
The above filters by the product's permlink.
RE: Hive Open Market launch: You can now sell goods and subscriptions on Hive in exchange for crypto.