Not a long time ago, you could easily go either for a free app or a paid app. But these day there are many free apps which may cost you more than paid apps, if you use the in-app-purchase (IAP) options available in the game/app.
So, how to tell if an app offers in-app-purchase or not?
On the Web version of Google Play Store, go to the app/game page, and check the line below the "install" and "Add to wish list" button. if the app/game features a IAP, then you should see the "Offers in-app purchases".
Otherwise the app/game is completely free.
On the Google Play Application, browse to the app/game you want to install, below the applications screenshots, look for "In-app-purchases" to see if it offers IAP or it is really free.