How do I distinguish apps with in-app purchases from completely free apps on Google Play?
I'd say we've waited it out – as meanwhile this issue seems solved:

 
Screenshots (click to enlarge)

As the first screenshot (top of the page when visiting with the web browser) clearly shows: If an app offers in-app purchases, it is marked such (look where my arrow points to). At the bottom of the page, you can even see the prices per item (second screenshot).