The desktop version of Google Play now shows if an app offers in-app purchases, finally
Now, if you browse the desktop version of Google Play and stumble upon a freemium game or an app, you will be notified that it "Offers in-app purchases" – this exact text will be displayed next to the list of the compatible devices, below the "Add to Wishlist" option. However, unlike iTunes, Google's app store will neither show you the type of these in-app purchases, nor the damage to your wallet that they might cause. Many users state that they want such information to be displayed, too, and only time will show if Google will overhear their pleas.
source: Android Police via Android and Me