Not realy a tool, but was stuck today having to hook my boat and nobody at home to guide me so...
Attached the GoPro to the boat in such a manner that I can see the coupler. Then synced to my phone.
Then I could simply watch the video steaming on my phone and line up the Pick-Up and the trailer perfectly.
Just be carefull as there is about a 2 second delay for the video to show on the phone.
I did the same to put the boat back into the garage. Simply fitted the GoPro to the back of the boat.
Perhaps it could have some other applications where you need to do something on this side of the work bench and have to see what is happening on the other side..