The adoption of Google's Play Games collection of APIs and tools has been rapid, and very welcome among Android gamers - it's one of the first things that gets requested when a new title on the Play Store omits it. Now it's a little easier to tell which games support Play Games tools and which ones don't, without having to download them first. Users, including those of us here at Android Police, are starting to see a Play Games logo appear prominently in the Play Store app description of those apps that support the system.
For example, in the Mortal Kombat X app description, the gamepad logo appears right above the "read more" link. Open up said link and you get a short description of which Play Games features are supported, in this case automatic sign-in, leaderboards, achievements, "and more." An exhaustive list for those games that include more would be nice, but this is still pretty useful. Compare that to Star Command (below), a barebones Android port if ever there was one, which has no support for Google Play Games.
You may have been seeing the icon for a few weeks already - Google tends to roll out Play Store changes gradually on a server-side basis, without explicit version updates or changelog. If you see something different, let us know in the comments, and please mention what country you're located in as well.