Virtually every airline will tell you on their bookings pages. If you’re flying in the front of the plane, there can be major differences in the hard product between different planes of the same carrier. Qatar Airways kept some crummy slanted recliner chairs in business on their A340s for a long time after the A380 had a great fully-flat bed and a bar onboard, and would fly it on the same route. FinnAir have some great seats on their Asian fleet A350s, but you’ll have a nasty shock if you fly Tokyo-Helsinki-New York as they have some truly dated and shabby A330s flying the latter part.
But: it doesn’t form part of the contract, so no complaints if you get shafted with an old plane.