If you box up and take all your gear, including the weight/bulk equivalent of a week's food and a day's water, to the store, that will help. As mentioned, there's a lot of difference between weights or sandbags and real gear.
Pack fit is almost as individual as shoe fit. The pack needs to fit your body, fit your gear and be comfortable for you to carry with your gear inside. For example, while Heather loves her Osprey pack, I have not yet found an Osprey pack (either unisex or woman-specific) with a hip belt that is comfortable for me. A "hike" of at least an hour around the store with fully loaded pack will help you determine if the store staff knows what it's doing.
Make an advance appointment with the store (for a non-crowded time) and insist on their most experienced pack fitter (when I go in, REI always gives me the most inexperiened teenagers unless I scream). You'll of course have to fit his/her work schedule. Tell them you're going to bring all your gear with you and that you need to try the pack with the gear. As a fringe benefit, you will hopefully also get some info on the best way to pack it.
Before getting any farther into gear selection, please check out the articles and gear lists on the home page of this site, left hand column.
There's lots of useful information there!
I ended up ordering a pack from one of the many "cottage" manufacturers that sell exclusively online. I was fully prepared to pay return shipping charges for half a dozen packs if necessary to get what I wanted. Amazingly, the first pack I ordered fit me as though it had been made to order just for me! I had my gear and the food/water equivalent all ready and loaded up the pack as soon as I took it out of its box. I "hiked" around the house for two hours--most boring hike I've ever done, but worth to be sure the pack was comfortable while I could still send it back. I'm still using this pack after 8 years and have no plans to replace it.
Even though I didn't buy a pack at the local stores, the several hours I spent at two different stores was worth it to learn what a comfortable pack feels like. The only one there I found that was comfortable weighed 7 pounds. The pack I ended up with weighs 29 ounces and, for what I carry, is fully as comfortable as the 7 pound behemoth!
Oh and another hint--don't go in just after a big sale. I visited REI in early April, forgetting that their big sale is generally the last half of March. All the lighter weight packs were out of stock!
Of course, as we say in the backpacking world, YMMV--Your Mileage May (and probably will) Vary!