In Reply to: Re: Clocking posted by Victoria S Dennis on September 20, 2008 at 14:16:
: : Origins of "to clock someone"?
: Clocks have faces, so "clock" became slang for "face". Thus, to "clock" someone is to hit him in the "clock". (VSD)
And from the same root, I think, comes the slightly different meaning of to recognise, or pick out, someone.