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Apple has a small note saying "Offer In App Purchases" below "Free" button. Though you have to view the app page for that.

In iTunes & Web, there's a section for popular IAP under the app icon.

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Google has that too on the app page, right below the download button.

As far as I can tell though, there's no way to tell what the IAP are on Android. And because most of them involve buying "gems", I just don't bother with free any more.

It also shows up as a small + symbol below the installation in the app page and the search page. Anywhere you see a price, you can also see if it has IAP.

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