The official introduction of the a/c on domestic routes is with the fall schedule effective Sept 6. Some of the flights so far include:
DL 582 ATL-SFO
DL 1648 SFO-ATL
DL 1676 SEA-ATL-MCO
DL 582 MCO-ATL
DL 1648 ATL-FLL
DL 1115 FLL-ATL-SEA
DL 1874 JFK-LAX (start 9/12)
DL 148 LAX-JFK (start 9/12)
DL 1763 JFK-SEA (start 9/13)
DL 1514 SEA-JFK (start 9/13)
DL 1150 JFK-SFO (start 9/14)
DL 1448 SFO-JFK (start 9/15)
DL 725 JFK-LAS (start 9/18)
DL 726 LAS-JFK (start 9/18)

Also later in September it looks like some other routes will get them too:
DL 446 ATL-LAS
DL 724 LAS-ATL
DL 649 ATL-SNA
DL1778 SNA-ATL
DL1085 ATL-PDX, etc.

Another way to tell is on delta.com in the Flight Schedules section, if the a/c type for the flight is listed as 757-200 instead of just 757, then it's the newly-reconfigured ones.

Apparently, they are currently working on the first one - the a/c should be flying this week or next (ship 6717 IIRC)