How to know app was purchased in Android?

I have to implement in app purchase in my application, so I used below link

http://developer.android.com/guide/market/billing/billing_integrate.html

Almost everything is working. But I have doubt that is how to know if in app product was purchased or not for particular device before purchase the app.

Because of 开发者_JS百科my requirement is to show my in app product to purchase.

If particular device already purchased my in app product there is no need to show my in app product.

How can I know app is purchased or not before purchase the in app product?

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The purchase information is stored in PurchaseDatabase class. From this class I am checking whether application was purchased or not.

For more information see this link

http://developer.android.com/guide/market/billing/billing_integrate.html#billing-implement