I've done a search on this, and there's no solid answer for MW3.
Looking at the bars in the lobby right at the beginning of the match to see who has the 4 bars first only works in Black Ops; it's NOT obvious in MW3. I've tried this method so many times and it's not a good method to check for host in MW3.
I see so many comments in threads where people talk about pulling host 90% of the time, or X percentage of time, but how can you be so sure? I don't want to figure out if I'm host by leaving the match to see if there's a "Host Migration" message.
I've read other theories, but none of them really work. I don't want to assume I've pulled host because I'm experiencing lag.
A definitive answer would be much appreciated.
if you are the first to connect to the lobby, then you are 100% without a doubt the host. You usually have to choose your class really quick to figure this out.
I've also noticed that in general, games where I connect really fast, the guys on the other team that are still connecting are the ones I seem to have the hardest time killing. They are the ones that seem to skip around the map, commando knife me, and eat half a clip from me without taking much damage.
Pretty much the only way is what dlundebjerg said. Not always easy to figure it out.
Knife the wall if it makes the 'ding' noise as you do it your the host
I am also starting to notice if the scores reset to 0 the top person in the list is the host. At least it seems that way.
I have been playing infected alot lately and the only way that I have noticed is through this game mode. Since your loadout is preselected for you you go straight to the map. If you find yourself on the map and the score says your winning 1-0 and then jumps to 18 you are the host. I also noticed that I don't hear the countdown or background music prior to the match when I host.
I haven't seen an absolute way to figure out if you're the host until you leave the room. Sometimes it's easy to get confused when getting your ass kicked by either a good team, a 3 bar crap connection (SOME 2+ KDR players have this), or someone is downloading too much junk on your home internet. I've noticed also that if the host has a crappy connection, his team losses pretty badly because apparently the enemy team knows how to glide around corners.
All you need to know is that if almost all enemies kill you with one bullet after they popped on to your screen for about a second; it will obviously frustrate and irritate you to a point that causes you to rage quit. After rage quitting, you'll know your answer (host migration or not). If no host migration happens then the other options mentioned are a good possibility.
I think that many people confuse some of the things mentioned above. But I can guarantee you that becoming the host constantly for people with very fast internet connections happens a lot. I used to happen to me constantly in Black Ops but not as often in MW3. It would happen to a point in which I would practically leave the lobby after the first or second match.
You can also check the open connections on your router.