Just stand on the side of the bike with the peddle at six o'clock. Hold the bike away from you, e.g., about 30 degrees, and push down on the peddle.

See how much the flex is? That's normal--really noticible on a larger steel frame.

It's not a bad thing and makes for a comfortable bike for many riders but maybe it's not for everybody. People that don't like flex might refer to a large frame steel bike as being a bit whippy. Mine is. Maybe try an aluminum frame and see if you like a stiffer ride better. I like the feel of steel so it's not a bad thing to me.