Yup, go to the reviews and choose to "Write a review", if it shows you a previous review you've written or allows you to start typing a new review it means you have previously purchased it.
If you've never purchased it you won't be allowed to write a review
If you just want to check quickly what you've paid for a certain app just type the app's name into Mac Spotlight (maybe add "buy" if you have an account @ Click&Buy) and—voilà! There is the invoice mail in the search results.