Xray. No way to tell for sure it's fractured without an xray. See a doctor.
X-rays. The best way would be to see an orthopaedic surgeon and to have an appropriate physical exam and x-rays.
Image study. The fracture, or contusion of patela, or even a sprain strain of knee joinit can have similar pain and confused symptoms. A X-ray will rule out boney problem for most part.
XRAY. X-ray will be able to differentiate a fracture from a bruise.