Thank you flying_pig for pointing out a problem with A320 usage.
There are only 9 longhaul A320 aircraft in the fleet at the moment which is totally inadequate to service all the longhaul destinations. So several longhaul destinations are serviced by shorthaul aircraft.
Which type is scheduled for your flight can sees on the seat map under Manage Booking. The Shorthaul Y cabin starts with Row 13 whereas Longhaul starts at row 14.
Can't see the Y section then I would suggest looking at the recent flight history of the destination on TheQatarsource.com where you can see flights, with aircraft registration number, for the last 4-5 weeks.
The longhaul registration numbers are A7-AHP to A7-AHY except A7-AHV which does not exist.
Since the biggest difference is in the J cabin remember that Longhaul have lie flat seats whilst all others have sloping seats.