Subjective Examination:

Severe injury (acute trauma) or ongoing (chronic) stress on the fat pad below the kneecap. This often occurs during activities that require full bending or full extension of the knee. The fat pad is pinched between the thigh bone and shinbone, and becomes inflamed, causing pain.
Patient involved in sports that require repeated, forceful straightening or bending of the knee (kicking or jumping).
Pain reported below the patella.
Patient reports that pain that gets worse with physical activity, including sports or when completely straightening the knee.
Objective Examination:

Swelling of the knee (sometimes).
Tenderness and swelling (sometimes) on either side of the tendon connecting the kneecap and shinbone (patellar tendon).
Hoffa’s test: With the patient in lying with their knee bent, the examiner presses both thumbs along either side of the patellar tendon, just below the patella. The patient is then asked to straighten their leg. Pain and/or apprehension of the patient is considered a positive sign for fat pad impingement).
Observation of Genu Recuvatum (hyperextended knees)
References

Image from OpenI – Licensed by CC
Image from OpenI – Licensed by CC