I have no way of knowing although I suspect a few have. On the other site I was on you always knew if somebody blocked you because you could try to click on their profile but got a note saying they blocked you. How do you know on GAG?
There is no indication until you try to contact them or post on anything of theirs. If you try to share on their question, you'll see a message telling you that either they blocked you, or you have blocked them. If you try to comment on their opinion, same message will appear. If you try to send them a private message, same message will appear.
There is no other way to tell at this time.
This information can always be found in our FAQ, please check it out :-)
You only find out if you try to answer their question or comment on their answer, or message them. A message at the top of the screen will say you're not allowed to post because they either blocked you or you blocked them (when you block someone, you can no longer comment on their stuff either). That's the only way to know, since block lists are private.
I know I'm blocked when I try to submit an opinion and it says I've been blocked. You can't message someone etc. I'm not sure if you can't still see their name (I'd doubt you can, the person who blocks a person may never see their messages).