Yeah, building on IV_Tha_Killa's post, there are a few ways in determining whether you are the host on PS3:
1) You start the game before anybody else (you normally go from Synchronising Game Settings to Waiting for other Players really quickly)
2) You have full connection when all the other 11 players "red bar" at 2-3 seconds before the game starts.
3) You rarely lagg within the game unless you start downloading on your own internet.
Although sometimes, this is not always true.
If one of your friends who lives close by to you is host, you still may have full connection. This is because MW2 bases it off of how close you are to the host (as I've heard, don't quote me on this please) so surely, if your the host, you are the closest to the host as you can be.
It's a stupid way of telling if you are the host (and really, its stupid) but if you leave and it goes to host migration, you WERE the host.
Please +rep if this helps.