Some sites show it on the booking page, and if not, there are various sites* on which you can look up the flight number to see what aircraft type has been used

Some sites show it on the booking page, and if not, there are various sites* on which you can look up the flight number to see what aircraft type has been used historically. Sometimes several different types are cycled on a given flight (e.g. Boeing 777 and Airbus A350), but for many flight numbers it's always the same aircraft type. For example, American Airlines has tons of both 737 and A320 aircraft, but AA858 seemingly always uses a 737-800 (B738): https://www.flightradar24.com/data/flights/aa858 https://www.flightradar24.com/data/flights/aa858 * e.g. flightaware.com and flightradar24.com