How to tell if a J150 motor mount has been installed backwards on the arm

The motor mount on the J150 arm can be installed rotated 180 degrees from its correct orientation. Because the mount still seats and the bolts still line up either way, it's easy to install it backwards without immediately noticing.

Installed correctly, the drone functions as it should. 

Installed backwards, the drone will act erratically, shake, spin, and possibly crash.