Hotmail messenger

Might work - or might be an email harvester, but worth a look at least.

Basically you try and add someone, if they appear online then, but they appear to be offline to you, you've clearly been blocked.

Make sense?

Thanks! :)

I'm not sure how it works, but I've heard that Gaim will let you know if someone has blocked you. Gaim is a free replacement for MSN Messenger, Yahoo Messenger, AIM, etc... It lets you login to all of them at the same time.