Most often, this becomes a more significant issue when looking at international flights, especially when an airline has re-fitted some aircraft with newer (potentially lie-flat) seats while other aircraft are flying with older seats.

Domestically, there's usually minimal variance (although some international aircraft with lie-flat beds so fly domestically), so it's not quite as much of an issue. As ozdude mentions, it really depends on the airline, as to (a) how much information they divulge about your specific aircraft and (b) the variance in seating options among their aircraft.

If you have specific questions about a specific airline and/or flight, I'm sure someone would be happy to help you look into the details.