To look behind oneself, typically by looking over one's shoulder. "Six o'clock" is a military term referring to the area behind oneself. Make sure to check your six, in case someone tries to follow you.
check your six
tv. look behind you (where there may be danger). (see also on your six. Refers to six o’clock as being behind one, as if one were facing twelve o’clock.) Check your six, bud. Some gangsta’s getting close to your wallet.