I'm confused, do they only show upcoming IMAX movies at an IMAX?
IMAX projectors run 70mm film that is 10x bigger than 35mm used in traditional theatres. Some multiplexes have a 35mm projector next to their IMAX projector and could play any preview they wanted to in theory. I don't know if any do this or not. They do play 35mm films on IMAX screens from time to time, but there is more to IMAX than the screen. If you see a preview in front of an IMAX film, its a very, very good chance that it's going to be an IMAX release. One of the reasons there are less previews on the front of IMAX films is because there are far fewer IMAX releases coming out. 10 IMAX releases in a single year would be a lot.