Airlines often publish standard seat pitch information on their websites but if you want something more specific, SeatGuru is a great resource for researching a specific plane’s layout. You can search by airline and flight number or by aircraft model (this information can usually be found on your reservation or digital boarding pass). SeatGuru breaks down each fare class by number of seats, seat width, and seat pitch and also provides an interactive map of the plane’s interior complete with passenger reviews, photos, warnings, and tips.
Google Flights also provides the seat pitch for a particular flight, and notes whether it is average, above average, or below average with its search results.