Not underneath the heatsink, below the heatsink Slightly closer to the USB connector than the edge of the heatsink. The reset testpad is the pad on the back labeled "reset".
A visual inspection doesn't show any clearly damaged areas, but I've included more pictures just in case.
https://imgur.com/a/tOHbYI measured the voltage using the reset testpad and these were my readings:
VCORE: 0.55 V
1V8: 1.80 V
0V9: 0.48 V
RESET: 1.61 V