How do I know when migration is complete?

Wi-Fi is your problem. I know that the instructions say that you can use it, but do not. Many Wi-Fi connections are just too slow and can not accommodate the amount of data being transferred. The possible result is corruption of the data. Instead, use a wired USB cable between the computers. The transfer should be done in an hour or less and it will, as noted, tell you when everything is complete.

Welcome to the Apple Support Communities.

You should see a message saying it is done with no progress bar. What do you see and how are you transferring your data?

Jack