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- from the airline's own website. Using Qantas as an example, check the 'timetable' section, plug in your to/from cities, bring up flight no. for your flight and there will be a clickable link identifying the a/c type
- from something like checkmytrip (for airlines using amadeus eg. Qantas) - plug in your surname and booking ref number and you will get a/c type and possibly a seatplan which would also identify config as well
Probably other ways to do so also. Configs vary and there are ways to check what the config would be , eg. Expert Flyer tool (a paid service). Of course, different a/c may be substituted for various reasons.