Go to the airline's website. When making a reservation, look at the different classes of service.
Go to http://www.seatguru.com
That site lists all the seating configurations by aircraft on just about every major and regional airline flying. Gives width and pitch information too.
If you are talking about "classes" of seats not just by coach/economy, business, first ect and are interested in Y,K,Q,M ect, those are fare catagories, not seat catagories. For example in coach you may have all of those fares and as the plane sells out only the Y fare will be available as that is the code for a full fare, unrestricected coach seat (walk up price).