72s are easy to tell apart from 73s and 74s. 72s have the bumpers close in to the body, like the 70s and 71s did, and they used the same thin chrome bumper guards that hang below the rear bumper like the 70s and 71s did. For 73s and 74s the bumpers stick out about 2 inches further out from the body front and back due to the "5 MPH Bumper" federal regulations, and they came with the big ugly rubber bumper guards that many people removed over the years. You can see those bumpers sticking out away from the body from a long way away.

You could tell the difference between a 73 and 74 if its got the "340-4 barrel" or "360-4 Barrel" emblem on the hood, since 73 was the last year for 340s and 360s were available in 74 only, but that also could have been changed out through the years, so it's not foolproof. Other than that, it's tough to tell the difference between a 73 and a 74 without looking at the numbers.