An email sent by Google to Android developers reveals that the company will require that all apps on the Google Play store which are ad-supported are labeled as such. The new policy will go into effect, and Google Play listings will show this information, on January 11, 2016.
According to the email, developers whose apps contain ads have to sign in before that date and update their store listings to reflect that.
The move is reportedly an extension of one facet of Google's current "Designed for Families" program to the overall Google Play Store. Designed for Families is an opt-in program with stricter requirements than merely listing whether apps are ad-supported or not, however -- it also requires they're ESRB-rated with an E or an E10+, which is not required for the general Google Play store.
Developers who want more info should check out the full email from Google (you should have it already, but if not, a copy is available here) and, if necessary, access the Google Play developer help center.