Google's Play Store will mark and reveal apps that have ads
Additionally, to avoid any misconceptions about “freemium” apps, users are always notified when a certain app contains in-app purchases. It seems, however, Google will build on that, as a new requirement for developers claims that they should declare whether their apps use ads or not (this includes banners, interstitials, native ads, or in-house ads.
What does this mean to you? Well, for one, you will be able to quickly determine whether you can find a free and ad-less app for your selected category, before finally breaking down and downloading the least ad-infested one you could find... or shelling out $1 for a Pro version of whichever one looks best.
For the currently active apps, developers have until the end of January 2016 to declare whether or not their products have ads in them.
source: Google via Ubergizmo