Some people do this with a photo, like this:

https://steemit.com/noisy/@noisy2/verification-of-my-two-accounts-noisy-and-noisy2

others do this with already existing social media account:

https://steemit.com/art/@zuch.media/part-time-job#@zuch.media/re-steemcleaners-re-zuchmedia-part-time-job-20160922t133645035z

Basically, introduction post is a best place for authentication :)