The specific indicator GM advises is a vibration peak around 24 Hz (+/- 2 Hz). This is detected with a picoscope, a pretty specialized tool.
Unfortunately there can be a lot of car vibrations in this frequency range. Probably the best "seat of the pants" test would be for someone who has had the problem to drive the car.
Here's the bulletin:
https://gm-techlink.com/?p=11127