I think you would need an additional column in the topics table to reflect the original forum id so that it could be compared to the current forum id, and base your choice of icon on whether the two were the same. Of course, you would also have to update that field when creating and moving the topic.
--Jördan It's a wasted day if you don't spend at least part of it flying upside down.
I had suggested a MOD a little while back that had something to do with e-mailing all users that had posted in a topic if that topic was moved. I was hoping someone could come up with that.