Instead of a couple of gallons of water, take your gear, including food and water for a typical trip. That way, you not only get to try the pack with weight, you'll also be able to see how your gear will fit in it. Besides, sandbags (most stores use them) or gallon jugs may get the weight you'll carry, but they will ride in the bottom of the pack, giving you a very unrealistic idea of how the loaded pack will feel. Your own gear will tell you how the pack will actually ride.

One thing: if you spend a couple of hours loading gear into and out of packs, and find one you like: buy it at that store, don't go home and order it online just to save $5 or $10 in sales tax. Treat the local vendor (whether it's a small independent or a big-box like REI) the way you'd want to be treated.