Just make a dummy booking through this site:

http://www.flyaow.com/classcwt.htm

Proceed right up to "select seats" and you usually are shown a seat map with available seats shown accordingly. Sometimes it won't give you that option - in which case you are snookered.

The following site:

http://www.flyaow.com/classcathay.htm

is good for CX flights too.