You can check previous flights
http://flightaware.com/live/findflig...stination=CYYZ
I just looked at a few and saw some from D, E and F. Toronto is going to be treated very similarly to a domestic airport for many things due to the fact that it has preclearance. I am not 100% sure but I think the last time I flew there from ATL we left out of A or B. I know that the last time I went there through MSP we departed from concourse C (domestic), if that helps any.