TedT, 
I just found this out. If you want to know if someone on Threads can be followed on Mastodon, you can type the following into the terminal:
curl "https://threads.net/.well-known/webfinger?resource=acct:potus@threads.net"
For the POTUS account on threads.
If you get back a JSON object that doesn't have an error in it, you have someone you can follow.
This means it's fairly easy to write a local script to see who has turned on federation over on Threads.
