How does anything know? 
This isn't a standard feature offered by most devices--and being unfamiliar with Vera (aside from a brief one-week experiment before eventually moving to Hubitat), I'm not even sure exactly what this means. My best guess is Zigbee or Z-Wave devices that have "fallen off" your network. In most cases, there's no good way to know aside from guessing based on things you think should have happened, like the Wake Up Interval in the case of Z-Wave. In all cases, "hasn't had activity within x hours" is probably the best you can do. (This is what I think SmartThings does and what Home Assistant's ZHA component seems to do, not always accurately--both built in to those platforms/integrations).
Hubitat doesn't have anything like this built-in, but if that's what you're looking for, it's easy to set something up yourself. The most popular option is probably the community Device Watchdog app. I also wrote one called Device Activity Check that I use for myself. You can find both here on the forum or with Hubitat Package Manager (another popular community app) if you're using that.
And if that's not it, it's probably still possible...somehow...most things are.
