-- Last Updated: Dec-13-12 2:36 PM EST --
You make a good point that picking a paddle "off the shelf" is a tough way to know what you are getting. Fortunately, I think if a person pays attention to such things, he can get a fair idea what sort of flex is there before the purchase. I have two "identical" cedar paddles purchased many years apart. The second one was a quick replacement of the first, which is broken (until such time as I take on the rather big project of fixing it). The new one turned out to be MUCH too flexible, but it turns out that the difference is perfectly apparent simply by grasping each paddle the normal way and forcing the tip of the blade against the floor, in a motion similar to pulling the blade through the water (I actually can test the broken paddle this way too because the broken spot only yields in one direction).
In the "old days", a person experienced with high-quality road bikes could get a pretty accurate idea of the flex in a bike's frame by standing alongside it while grasping the seat and handlebars, then pushing sideways on the crank hub with one foot. I bet a person who wants to, could develop a feel for paddle flex in similar fashion.