In the newest release of MW3 on XBox, and presumably all platforms, I get the message "Warning: Searching for DLC is taking some time. Do you wish to continue?" Does this mean I am host? I see the message, but at least one other person in my party does not, and I'm trying to figure out why. I often struggle with lag, so if it is an indicator of host, that would be awesome.
Yes, when MW3 says that it is searching members of your party for DLCs, that means that you are the party leader, otherwise there would be no need to know if other members of the party had DLCs, because the party leader would figure that out. The host hosts/runs the game on their machine, so they lag a lot, but they do not have the DLC message, because they aren't predetermined as the party leader.
Hope this helps!