Try looking up the flights on
http://www.delta.com by pretending to make a reservation. As it gives you the flight choices for each leg, you will see all the way at the right edge of the chart is a button you can push for each of teh flights to pick seats. It will let you know how many seats are open on the flight. This should give you some indication at least, of whether there is any chance of getting on. Also you can use
http://www.trip.com and pretend to book, as their engine gives you a number of how many seats are open ion each fare class.