how can i see if my phone is fully Unrooted

[Zero Temp]

Hi all . I have rooted my phone (LG G3) and because of a problem i was forced to go to my service provider (LG) since its still under warranty . So knowing that rooting voids warranty i unrooted using superSU and then i verified with root checker getting back the message "sorry root access is not properly installed on this device" . So i thought
that its all done and proceeded to use my warranty . couple of days pass and they called me and said that root was found on my phone and cant take it under warranty and i have to pay if i want it fixed which i am not gonna do cause its 200 euro to fix the motherboard. I asked them if i can do anything and they said that if i remove completely the root i can go to them again and they manage to fix it for free. So i flashed a new stock ROM to be sure this time is fine. Now what i am asking is can i somehow check in depth if its rooted or ever been rooted like traces and stuff . is there any program that lets me do so? i tried to find the program the technicians used but couldn't find anything.
Thanks in advance for all the answers and for your time .

 

[anon(632115)]

It should be clear of anything associated with root. What does root checker show?

 

[Zero Temp]

root checker says it cant find root access . i am just asking if there is an other method to verify that

 

[anon(632115)]

Having completely installed a new OS you are back to stock. I don't know of any other ways to verify although you could try an app which won't work on a rooted device. eg a banking app