From the flyerguide wiki...

What are the chances that *my* flight will have the new seats?
If it's a 763 (Boeing 767-323ER): 100%
If it's a 777 (Boeing 777-223ER): ~96%
If it's any other equipment type: 0%
(Information as of July 2008)

[edit] Will *my* flight xxx on xx/xx have the new seats?
Certainly if flying on a 763, and excellent chances on a 777; only two 777s are flying today (mid-July 2008) with old Business seats, according to recent posts, so your chances of NGBC seats are a very good ~96%. London flights anecdotally seem to get first priority on new seats, South America and India last.

Otherwise, there's no way to know until the morning of the flight (see below).

How can I tell if a flight has the new seats?
First, the flight must be operated by a "763" (Boeing 767-323ER, which have all been converted to NGBC,) or 777 (Boeing 777-223ER, all but two converted,) plane. Secondly, you won't know until seat map assignments are updated sometime overnight local time of departure. After the update, you should pull up a seatmap at AA.com. (N.B. some 777 seatmap failures have been reported.)
For a 777, if the seat map shows seats for 8HJ (instead of the lavatory), then it's an NGBC plane.

Note that equipment substitutions happen all the time, at hubs even up to the last minute, and that seating type is not guaranteed if you are flying a 777 summer of 2008.