This article explains how to see the list of mobile devices that have currently logged in using Now Mobile App.
There is no table in specific which keeps the login information of all the now-mobile logins. But there is a table called "sys_push_notif_app_install"(when a device is able to receive push notifications, there would be an entry here) which our customers are using to tract agent login. Every time an agent logs into the now app, the active would be set to true(And push application will be set to ServiceNow Request Application) and it will give additional information about the device here. Last registered would be the time the agent logs in. This should help you track down the agent login information.