Like any flight, check the travel line religiously to see how the flight usually fills up on certain days...
The only difference with Express is a lower dependability rate (better or worse with certain operators) and the chance of weight restriction (an example being 50 seats with the ability to take only 46).
Many of the routes we have Expressed we have lost customers on. Customers like an RJ from a hub to a Paducah or a Peoria, not an Atlanta or a Minneappolis.
Not too sure about Toronto specifically. As usual, have a backup and a basic knowlege of which hubs we fly there from, as well as the hub-to-hub flights which can be tricky.