I remember two tests that would indicate a possible stress fracture:
First, the hop test. Can you hop on you left leg? When I had the FNSF it was nearly impossible. The pain was intense. I could barely get a few cm off of the ground and after one landing I gave up.
Second, the fulcrum test. Sit on a chair or bench and place your fist under your left leg. Now, take your other hand and apply force to your left knee. You can move your fist, the fulcrum, along different points under the leg. I could find an exact spot where the pain was unbearable.
If it means anything, my stress fracture pain was most intense at the beginning of the run, then it would feel better, and by the end of a 45-60 min run it would be so intense that I'd have to stop and limp.
If you haven't gotten checked out, I recommend going to see a doctor. You are most likely going to need to get an MRI. You don't want to mess with a FNSF, all kinds of nasty things can happen, like messing up blood supply to the hip joint, necrosis on the femoral head, and just a really hard injury to heal. When you think about it, you are putting forces on your femur even when at rest, standing, sitting at a table or desk, etc.