How to prove the authorship of the site?

Question

Freelance

I live and make websites in the USA.
Fame and national recognition overtook me suddenly: for the first time at once, 3 (!) Russian (!!) one-day web studios placed on their portfolio pages a link to a site made by me. The specifics of freelance work is that projects and customers are constantly changing. The portfolio has one of the decisive roles in negotiations with the next customer.
How to leave information about the original author of the site, so that later the new Vasya Pupkin does not appropriate my merits?

• Prescribe Meta Author or comments with attribution?
  
• With the consent of the customer, add copyright to the footer?
  
• Throw "Easter eggs"?
  

I understand that it is not a problem for a true professional to prove his authorship in a couple of minutes, but the customer is far from always a knowledgeable person, and therefore a moment of distrust can easily ruin all the negotiations.

Answer 1

there is such a thing for these purposes humanstxt.org/
humanstxt.org/humans.txt
www.google.com/humans.txt

Answer 2

• Add sites to Google Webmaster Tools or similar tools of other search engines with proof of ownership and show to the client.
• Add a footer to the 404 error or other service pages, where the client will not worry.
• Put a file, for example, /author/mycompanyname.html and write it in robots.txt (so that search engines don’t find it and put it in the index), and on your site give links to this file:
  User-agent: *
Disallow: /author/
• In the same robots.txt, you can add comments (this certainly won’t hurt anyone), and on your site give links to robots.txt:
  # About this site and the author: ... link ... e-mail ... phone ...
User-agent: * # comments can be placed at any line after sharp symbol
Disallow:
• // Write comments in javascript
• <!-- Write comments in html -->

Answer 3

You can use the referrer to determine who came to the site. And, if they came from well-known "bad" sites, send them to your portfolio on the page with this site. :)

Answer 4

I personally don’t bother with this problem, although I’m not looking for “ignorant” ones - there are a lot of problems with them.
Technically, footers are usually not given away, although I have hundreds of thousands of links from them :)
the meta is probably the most correct, but we must not conflict with the new Google policy with the definition of the author of the content. (something I didn’t understand, maybe I’m wrong here)
Easter eggs are fun and fast, but there may be a rejection of the client “what will be on my site?”

Answer 5

Are you still not adding copyright to your sites? o_o