First of all, Happy birthday!
Knowing whether or not a player has purchased an item can be done using some events from the MarketPlaceService. << click the link for more info
There is an event called PromptGamePassPurchaseFinished that will also provide you with 3 values.
The player, the ID of the product, whether or not it was purchased.
There is also a ProcessReceipt callback which is more commonly used.