The tools people are using to find the Mastodon/Fediverse handles of their Twitter friends.
The first two are similar, they search for Mastodon handles in profiles (with false positives). The third one lets you specify an account, encouraging repeat visits to see if friends have added theirs.
1. Debirdifier
https://pruvisto.org/debirdify/
2. Fedifinder
https://fedifinder.glitch.me
3. Twitodon
https://twitodon.com
Use at own risk (data scraping possible). Revoke access after use.