Here's an interesting and thorough set of videos by Big Chuck Kleinhagen showing how to check ESCs for non-timing "Blinky" operation. Chuck, owner of Fastlane Raceway, is also the organizer for the Brushless Racing League oval racing series.

https://www.youtube.com/watch?v=UOu42ywJ21I
https://www.youtube.com/watch?v=-IVlGhX4IZs
https://www.youtube.com/watch?v=MlM4HIJjiX8
https://www.youtube.com/watch?v=xFOJYhF9JkE

I want to thank Chuck for doing these-- he is a really busy guy, but still took the time to record these for us. Since he is a very busy guy, please don't contact him regarding the videos; I'll try to answer any questions here, in this thread.

Part 1 shows connections to the sensor cable. Chuck is using an sensor cable adapter that I made for him, but it's not absolutely necessary to do the test.

Part 2 covers scope setup. You can skip this is you are familiar with oscilloscopes.

Part 3 shows the actual test. If you only watch one of the videos, this should be the one.

Part 4 is an addendum addressing what's important and what can be ignored on the oscilloscope display.