Prior contact from the host email address and ip. That generally involves followups. We verify everyone before allowing a post by a major company representative to stand. However, we do not require them to present an email adr at that company to users - they like to protect their company e as much as we do from email harvestors. Smaller companies we can't verify of course, but we are always aware and watch for any monkey business.
If you are specifically referring to our open invitation that has been answered by several search engines, I can verify that we've seen posts by official reps from Google, Altavista, several from Fast, About.com, and Looksmart (apologies if I have missed anyone). I'd like to say thanks to those companies mentioned above that have taken the opportunity to post here. We know it isn't easy and many factors involved.
I've been working hard to bring several more companies in to discuss issues and their services with us. We have been and continue to work hard to build bridges. Not for what it can do for us, but what it can do for the community as a whole.
I've also had contact and issued open invites to Lycos (mainly the fine techs in Europe with Terra), Excite, Northern Light, Teoma, and WiseNut.