Means:
Doman A
Domain B
both are redirected to Domain C.

But can I grab something as referrer? Like:
Hi. I came from A
or Hellow. And me came from B!

I don't need PHP help: everything is clear there. Apache only.

Well, if the browser sends the HTTP Referer header, then you could use that to detect cases where they came to Domain C from Domain A or from Domain B. Of course, if there is no Referer header, then you don't know whether they typed the address directly or used a bookmark, or whether they came from Domain A or from Domain B and the Referer header just wasn't supplied.

How robust does the detection need to be?

Thank you Darin.

I am really stupid (sometimes)... I have 6 domains and one of them is working one. So, in the root of each of them I am simple writing diff. directive as:

Domain A (root htaccess)
Redirect permanent / http://myworking.domain/index_A

Domain B (root htaccess)
Redirect permanent / http://myworking.domain/index_B

etc... is it good solution for managing several domains as one web-site?

:: How robust does the detection need to be?
It has to be unconditional of course because other domains has nothing (without htaccess error will be returned)..

I don't understand. If you're redirecting Domain A and Domain B to different pages at Domain C, then can't you check the server logs for references to index_A and index_B to see how many people are coming from Domain A and Domain B?