Cheaters?

I'm a 2nd Lieutenant (60) with a 12 Class Medic and a 12 Class Support, which is my go to classes. I'm not an experienced player and not as agressive as some players.
BUT...
How on earth is it possible for a corporal with a 0 class Assualt to take me out with one shot from a SMG MP18, after I emptied a Lewis on his ace...2X? Am I just crap or is there something smelly in the fridge?
Or
A bomber to see me hiding in a building, in Sinai, and bomb my ace 4X? Not just with a machine gun but a bomb. I tired myself in an empty game bomb a building from more or less the same height, with a buddy hiding in the building, and it took a few passes before I got near the target. My buddy is an experienced player and has much more nothches on his belt. He had a go, as well, and he got pretty damn close, but not enough to kill me completely.
When i sniff a "cheater" I generally look for another game, but it would be intersting to hear from other experienced players to hear how to identify a cheater.