Originally Posted by ClipperDelta
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.
This doesn't appear to be the case any more; even reconfigured planes are listed as just 757. However, the 6 vs. 7 rows of first class difference still stands.