In theory getting the foot inspected by a trained medical practitioner would determine this.  I also read somewhere that moving the toes in the above fashion (e.g. pulling them apart or wedging them with cotton balls) would INCREASE the pain on a stress fracture... but I doubt there are universal rules on all this and it depends where the fracture is actually located.  I have read about people getting X-rays that do not show stress fracture and then having to get an MRI.