Every time you make a purchase that applies a charge to your credit card, a receipt is sent directly to the email address you have registered to your Apple (iTunes) or Google Play account.

An easy way to check your receipts is to check your inbox for the email address you have linked to the account which was used to make those purchases.

If you cannot find the receipts in your email inbox, you can always check for the receipts directly on the App Store or the Google Play Store.

Android - Google
You can find more information on how to do this here.

iOS - Apple
You can find more information on how to do this here.