Originally Posted by
mnredfox
Where in ATL domestic is it then? After getting off the train from car rental, go where?
From the car rental train, follow the signs to South Terminal ticketing/check-in, and walk ALL the way down the hallway. You'll pass baggage claim on your right, atrium on your left, and DL ticketing on your right. Do not turn left to go to the security lines through the atrium (where all the signs will point you), just keep walking straight and you'll hit the south security checkpoint where SP and PreCheck are.
It's the arrow at the top right of this map:
http://www.atlanta-airport.com/docs/terminal/mt.pdf
If in doubt, any of the DL agents in that area and (I'd hope) most airport personnel can point the way if you ask for the south security checkpoint.
FWIW, that checkpoint does close at some point. My return flights often arrive after 10pm or 11pm, and when I come out that way the entire checkpoint is shut down.