Use this handy tool by @Luca@vis.social to find users in the Fediverse that you follow on Twitter:

I wrote a nano tool that tries to extract the #Fediverse accounts of your #Twitter followings: fedifinder.glitch.me/

It searches for the patterns @user@host.tld, user@host.tld and host.tld/@user in the screen name, description, location and URL field. It displays them to you in the correct format for easy copying as well as a CSV download that can be imported to #Mastodon.

Known issue: Mail addresses are included as false positives because people write their handle without a leading @: name@host.tld

Screenshot of three browser windows. The first one shows the fedifinder before logging in. Short description and login button. The second one is a list of found handles. The third one the settings screen of Mastodon where the followings can be imported.