While going hands-on with the Nexus 5X and 6P a bit earlier, I noticed something interesting in the "about" screen of both devices: a new field. It's called "Android security patch level," and what it appears to do is display the date of your phone's most recent security patch.
We know Google has been taking significant flack for Android security updates post-Stagefright, so it seems this feature may be in response to those criticisms. I didn't learn anything else about it, but it was definitely on the 5X and 6P units I used, and speaking to a Google rep, they suggested this feature would ship on the devices.
Presumably, this is another new addition to Marshmallow in time for the public release, so you'll likely be seeing it on older Nexus devices once they get their 6.0 update. More information is always better, and this at least gives you a sense of how often you're getting security builds. Also, yes: that date is in the future. Oops.