As from next year, ad-supported apps on the Google Play Store will start getting tags too show users which apps contain advertisements just as it already shows apps that have in-apps purchases.
According to droid life, Google has already notified developers to declare if their apps have ads or not. The developers have until the 11th of January 2016 to declare if their app contains ads, failure to do so would be considered a violation of the Google Play Developer Program Policies.
Here's contents of the email that being sent to the developers.
Hello Google Play Developer,
At Google I/O, we launched "ad-supported" labels for apps in the Designed for Families program. Today, we're announcing the expansion of this program to all apps on Google Play. Early next year, users will start to see the "Ads" label on apps that contain advertising, both inside and outside Designed for Families.
By January 11, 2016, you must sign in to the Play Developer Console and declare whether your apps contain ads (such as ads delivered through third party ad networks, display ads, native ads, and/or banner ads). After this date, the ads declaration will be required to make any updates to your apps. Please note that misrepresenting ad presence violates Google Play Developer Program Policies and may lead to suspension. You can visit our help center to learn more.
While developers are responsible for accurately declaring ad presence in their apps, Google may verify this at any time and display the "Ads" label if appropriate.
If you feel your app has been incorrectly flagged for containing ads, please contact our policy support team.
The Google Play Team