Checking if a user came from a 301 redirect

[tezmc]

Hi

I'm changing the domain name of my website, and will be redirecting users from the old site, to the new with a 301 redirect. Is there anyway I can check if a user has come to the new site through the redirect?

I want to be able to show a thickbox popup explaining why they have been redirected. I tried checking the referrer, but it seems to be blank.

Any help would be much appreciated.

Thanks

Terry