I know what causes sway back...but how can you exactly tell if a horse is sway back? I know with pictures like

but what about just a slight case of sway back?
I know in some pictures, Sonny "appears" to have a sway back, but I don't believe he does because in others it doesn't.

(yes I was holding the camera at an angle for some reason hehe)
I'll add more pics later...but got to run to work