How can we detect a self-reboot or shutdown fault on a cellphone?

How can we detect a self-reboot or self-shutdown fault on a cellphone in a few minutes if the cellphone's motherboard or any component has a fault?

For example, There is an android or ios cellphone and we want to check and buy it but we dont have too much time to check it and some phones have motherboard faults, they self-reboot or shutdown by theyself 1-2 time in 1-2 hours.But usually we have only 15-20 minutes to check the phone, in this case what can we do to detect any problem on phone in less time? (I've tried 3DMark and PCMark apps to check phone's problems but I'm not sure about it's enough)