The hypothetical user can be certain if they simply do a force reboot by holding power and volume, instead of shutting down using the power button long press and touch screen swipe.
If you suspect your device has been tampered with and need a guaranteed reboot, just let the battery run down and don't use it until it has.
Just a (overly paranoid) thought: a sophisticated attacker could simulate a drained battery and subsequent reboot, no?