Donation buttons are now part of the Curse Client v126.96.36.199, which is great news for authors (including myself).
As it stands, users need to update the addon in the client in order to see the new donation links, but once that's there, they're quite visible.
Screenshot of the client:
Because the paypal button in the example uses the "Paypal" (non-hosted button) type, it shows up as OneBag3 (from WowAce.com) when clicked on WowAce, OneBag3 (from Curse.com) when clicked on Curse, and OneBag3 (from CurseClient) when clicked from inside the Curse Client. This should allow you to know where your donations are coming from without just guessing.