While the translated article is posted, there will be a edit summary that mentions the articles is translated using Google Translation Kit. This can be seen in recent changes and history. In Tamil Wikipedia, once a user starts uploading such articles, we mark his user talk page and categorise him under Google translators so we can review his edits. The articles will also be categorized under Google translation project and have an info box at the top so casual users can know it is a Google translated article. For Wikipedias that don't monitor Google project actively, recent changes / history is the only way to know.
 

In Tamil Wikipedia, earlier they wrote that the article is translated from particular version of English Wiki article. Later they stopped it. So, they don't write any thing now. The translated article upload process is a well coordinated operation between translators and Google and I believe they have internal tools to review this. So, they don't need to write anything to prove they translated.

We are also in a process of writing a complete review of the project. It will have complete details, suggestions and best practices for such a project. Will post it in a month's time.

Thanks,
Ravi