Lots of DVD packaging now spells out what aspect ratio the film is being projected in. For example, MARY POPPINS is clearly labeled on its box as 1.66:1; RAGING BULL is 1.85:1; THE INCREDIBLES 2.39:1. Once you've started taking note of the dimensions, you can spot them on your TV or at your local theater if you just look at the shape of the image as it's projected (if the projectionist is showing it correctly, that is.)
Leonard Maltin's Movie Guide identifies those films that are shot in named widescreen formats like Cinemascope, Panavision, Vistavision, Super 35, etc. Lots of home video magazines and on line reviewers also give the information in the specs foe the videos.