A sideways look with a squint, commonly used when you know you're guilty and you're lying, when you don't know what to say, or indicating disbelief.
Person 1: Dude where are my keys?
Person 2: I don't know.
Person 1: YOU TOOK THEM DIDN'T YOU?! >:O
Person 2: <_< maybe......
Person 1: DUDE YESTERDAY I ATE A CAT! O_o
Person 2: <_<