For a future dated/timed flight, there's no way of being certain until you are at the gate, and can see what it is that you are boarding.
however, most (though not all) airline websites include the type in their schedule and booking pages, and flight retailers like Travelocity and Opodo will also list the 'scheduled' type. Not that this may be vague - for example, BA
often list the type as 32S
, which means it could be A319/320/321. Amadeus is a good source of this info, but again looking in advance will only tell you what has been 'scheduled', and there are plenty of reasons for that being different on the day.
Planning a journey around a specific type (which is more common than you might think) is never going to be an exact science, unless of course you choose airlines that only operate a single type, like FR
I travel, therefore I am.