I heard somewhere, probably on this subreddit, that it's a good idea to take a photo of the lock as proof that I checked. So after six months driving, I started to do just that.

On the liability front, it's a no-brainer. If there's a mechanical failure that causes a trailer drop, I can prove that I checked and confirmed a secure lock. That could save my job, avoid fines, and even keep me out of jail in the event of an accident.

And my phone auto-adjusts for lighting, with zoom, so I get a much clearer visual than with a flashlight alone. I got my first failure to lock three weeks ago after a successful tug test. It was only a 1/8th inch sliver of darkness on the right side of the locking jaw, but on the photo, it was clear that it hadn't fully engaged. If I'd only inspected it with a flashlight, I probably would have overlooked it.