How can you tell if it's a "breath powered leg press" or a non breath powered one? I mean for yourself you can tell, by feeling your body ( I would assume ) But if someone else were doing a breath powered leg press, how could you tell?
Well for yourself its pretty easy to tell, because you can tell when you push with your legs (as in load the leg muscles and extend by using the legs, I keep the legs totally relaxed as its hard not to continue pushing on out with them) and when you don't push with your legs, but the legs get extending out as a result of the breath. The breath portion has 2 components, one has to do with driving the dantien (Im not going to say more than this as its too difficult to try and explain online) and two has to do with conditioning via breath.
As for being able to tell, you could probably hook them up to a machine detecting electrical impulses and see the leg muscles are contracting or not. Its not going to be super visible unless they have a highly developed dantien.
Like I said in another thread, I think, one of Forrest's stupid jin tricks, is that you should be able to see a movable dantien: a ball of muscle under the skin that can move up/down/left/right. Now sitting here at my desk, and thinking about the angle of the legs on the leg press machine, i think it might be obscured when the legs are close into the body prior to beginning the lift.