One can manipulate a frame to flex when it's not moving all day long, but when you're actually out on it, it's a different story.
I had a SS Soul Cycles 29er with an EBB, and after I built it up I swore something was wrong, bc when I'd stand on one side to check for any bb creak, the whole triangle would twist. Nothing was broken or damaged, it's just the way a frame works.
Getting out and riding it real time is going to be the most accurate way to determine if a frame has flex.
Actually, some of it is the rear wheel. The BB in my Pista Concept seems a lot stiffer laterally when I've got my disc installed than with standard box section rims and spokes.