Welcome to Flyertalk deckem, bienvenue.

On the day of departure, when the inbound aircraft has left London, look at flightradar24 for the incoming flight and note the registration. Then match that up with the aircraft that have been updated using the helpful fleet guides here on FT.

I stand to be corrected but isn't YUL a 3-class route? This would mean that you'd only get the old WT/WT+ unless there was a sub for a 4-class Aircraft.

you can also use ftdashboard.net to see what aircraft have flown the route in the last couple of weeks.