How to test for mirror flop?

[Swoop1]

Having just seen this topic-

I wonder if mirror flop could be the cause of target drift on my C6 SCT when doing video runs on planetary targets?

How do I check for flop?

[Tomatobro]

I found that my Hotec laser collimator had a M42 internal thread so rather than using the expanding rubber rings to position the laser I screwed it onto the scope in place of the camera. This gave me a very rigid and repeatable laser spot position. This was on my 10 inch RC. I then moved the scope around on the mount to see what happened to the laser spot to see if anything moved in the optical train.

The same test using the expanding rings to locate the collimator suggested that internally things were moving but this was due the the small amount of sag the collimator suffered as its orientation changed.