I used to say that I rarely ran into boosters, but as soon as I switched to HC Domination, I see them in a lot of games. I guess because it's easier to get kills in HC and you dont see where they're getting killed (no death skulls).
I posted this in another thread, but the easiest way to spot a booster pre-game is to look at all the names on both teams. If you see anyone with a (2) at the end of their name, that is a split-screener and their partner should be on the same team. However, there is a way to get them on opposite teams and thats the easiest way for a booster to work. So, if you see that, then it's almost always a guarantee they're boosting. And if the player (2) is on your team, then you're in a world of hurt from scorestreaks because that's usually the one that gets killed.
What I've been doing a lot lately is, whenever I see this occur in the lobby before the game starts, I give a heads-up to everyone. I say something like "Just to let everyone know, player X is going to be boosting in this game with his split-screen X(2). So, we might lose this game if they call in a bunch of scorestreaks." Or if the player boosting for kills is on my team, I'll let the enmy know that we may have an unfair advantage because player X is boosting and they should report them. Sometimes, its nice to get on a team that rallies together against the booster and force them to leave. A few times, the boosters cut out before the game starts knowing they were caught. They still get reported, of course.
Just a tip for everyone to look out for.