I'm sure you're familiar with the countdown protocol, all the pre-flight checks, etc. These power up a range of subsystems, motors, etc, so that everything can be verified prior to ignition itself. The complete sequence takes a very long time. Under normal flight conditions, you can't check for absolutely everything (instrumentation is mass, and mass is the enemy) but there's still a lot. However, during an engine test, you can pack a lot more sensors in.
This is where you'd want to be spotting loose connections, pumps that aren't quite even, pressures that aren't as steady as they should be, vibrations that shouldn't be there or do not match expectations, turbulent flows, and so on.
At ignition, it takes between 3-6 seconds to go from stopped to 90% thrust. For humans, that's near-instant. For a computer sensor that's operating a million samples per second, that's 3-6 million readings. A computer performing a billion calculations per second shouldn't have much difficulty in comparing 3 million readings against model predictions and determining if both the values themselves and the rate of change at each point such a sensor exists are all good. Emergency shutdowns during those first 3 seconds are perfectly viable.
Vibrations are the ones that are likely the most interesting, because those are likely to change before something breaks, not sure how fast you can make infrared sensors, but that's also an area where things are likely to alter before point of failure.