They are verified on their pay to play sites such as AVN, NiteFlirt, Adultwork etc. Twitter is open to all, it’s not an adult site, it just happens to host NSFW content.
I'm quite aware. I didn't say ALL, I said SOME, and having your paid links available in ones bio is "verification"...if they're on those sites having the link visible is just a good suggestion. Have a nice one.