Looking for rate of change in the compass of 25 degrees or more per 0.1 seconds. That's about a 90 degree turn in a third of a second, which is not normal.
Most aircraft don't turn faster than 15-16 degrees per 0.1 seconds.
Faster turns can be a result of extremely strong winds, a crash into an object (example of a crash), magnetic interference, or a bad compass.
