Google Now on Tap is available in the latest Google app update, only works on Android M Dev Preview 3
Google Now on Tap will officially debut with the release of Android 6.0 Marshmallow later this fall, but those impatient enough to give it a spin are in for some good news. It seems that the latest update for the Google app, rolling under the 22.214.171.124.arm version name, has Now on Tap enabled.
However, you need to be running the Android Marshmallow Dev Preview 3 in order to make it actually work. The reason for this is that the Dev Preview 3 is boasting the final APIs as well as the official SDK for Android 6.0 Marshmallow, which are most likely required to make Now on Tap work as intended.
It is reported that the feature doesn't work on Dev Preview 2 or earlier. All of you eager to give it a try can head to the source link and sideload it on your devices.