They have signs above them. If it's Magic Morning, go to a turnstile that has the sign above turned to "Magic Morning". Otherwise I think they just say "general admission" or something like that.
Unofficial Guide says to line up at Gate 13, because the line at gate 14 is partially blocked by a tree, and feeds from the line at 13, so it moves faster. A couple people on here have confirmed that the line on gate 13 moves quicker.
But if you're one of the first in line at security, you can take your pick of turnstiles. Sometimes it's only even or odd, but if it's not immediately evident, ask a CM that is wandering around behind the gate.
Danauk, I think I answered this on that other thread, but just pick a security table-- at 6 a.m. you'll likely be the first or second one there. They'll open the one where the line has formed behind you, I'm thinking.