In all fairness, sometimes it can be tough to make a transition from one mob to a next. I find that in a lot of raids, when I'm trying to find the back of a new target through a hail of AoE, it can be a difficult thing to do sometimes. Also, as a tank I know that unless Blizzard goes live with the Borean Burrito for some serious bear AoE threat generation (undead say the smell reminds them of home so they flock to it, everyone else just wants to kill the source of it...***** you Consecrate), I'm always going to have to be mobile and adjusting position for swipe or to pick up runners.
That being said, what I normally do is watch the middle of the screen for the "You must be behind your target." message and if I see it, I run forward a few paces and then spin. If that doesn't put me behind the mob, I know that I'm at the side of it and if I still can't see the tank through the hurricane/blizzard fiasco, I'm pretty much screwed on the damage meter for that fight.