How to make sure mount isn't tilted?

I decided to join the Comma hype train and have a 3X arriving tomorrow. I plan to install the mount tomorrow and let it cure for 48 hours, but want to make sure I don't accidentally end up tilting it. What's the best way to do so? Should I use painter's tape or something on the outside of the windshield, or is that overkill?

EDIT: thanks everyone! ended up using a tape measure and making a horizontal mark with a Sharpie for alignment