There are multiple methods:
1) If your app if just on Android, you can use the IAP C2 plugin event - HASPRODUCT, and then invert it to show the ad. Thats what I did for the Android version of my app and it works fine.
2) On purchase you can set a variable to be changed, and then just check what the value of that variable is. I used this for my IOS app, as for some reason the C2 IAP wasn't working properly with itunes.
As you can see the first method is easier
The advantage of the second method is that its doesn't require an internet connection to make the check.
Hope that helps