I worked with Motorola in one service center.
Motorola enforced us to check every devices warranty status in a private webpage of theirs. Rooting voids and shows in the webpage.
As long as you don't root it with the tools motorola provides we have no way to check wether the phone has been rooted or not, especially if it's bricked. We would usually swap the board and that's it.
Actually, we could check it in the recovery, but Motorola doesn't say this is the way to check so we ignored that.
Have fun rooting your devices.