How is a foot stress fracture diagnosed?

X-ray or MRI. A stress fracture in the foot may not initially show up on x-ray. If the area still hurts 1-2 months later then a second x-ray may show some signs of new bone growth indicative of a stress fracture. Or you can get an MRI scan that is more detailed and may show the stress fracture site via swelling internally or on the surface of the bone.
MRI or bone scan. A stress fracture may not show up on an x-ray for 2-3 weeks and a stress reaction may never show. An MRI or a bone scan will show both of these early on.