A stress fracture is what most people know as a "hair-line fracture".  It is a small crack in the bone due to repeated stress.  When you are seen by a foot specialist for a stress fracture it may not be visible on x-ray at the initial visit.  It can show up on x-ray about two weeks later.  Radiographically, there will be a bone callous at the site of injury.  The appearance of the foot may look normal but sometimes minimal localized swelling may be present.
Other exams that can be used to diagnose a stress fracture are a bone scan, MRI, or CT scan.  In most cases these other modalities aren't necessary.  It is usually diagnosed clinically using a tuning fork over the injured site.
Treatments for stress fractures include modification in activities, post-op shoe, fracture boot, crutches, bone stimulator, or even surgery.  If a person lives a very active lifestyle and is allowed to continue with the same level of activities, it is possible to become a complete fracture.