As others have said, trying on several packs is ideal. Preferably without having to purchase and return (or sell) them. I went through that and it wasn't a cheap experience.
Another option to consider that doesn't get mentioned often is the frame packs from Outdoorsmans. They are heavy and don't provide a ton of shoulder lift, but I ended up settling on an Outdoorsmans pack as my favorite of all the high-end packs that I tried. I really like mine. Wilderness pack specialties, the manufacturer for Outdoorsmans, offers basically the same pack setup with slightly different suspension components and they call it the Yukon frame.
Most guys don't prefer them, but if you have a chance to try one out I'd give it a look.