Most definitive diagnosis of sinusitis is
Sinuscopy or Diagnostic nasal endoscopy is the most appropriate investigation.The classical feature is the presence of mucopus in the middle meatus or above the middle meatusA more appropriate investigation would be CT scan of the nose and PNS.Ref: Hazarika; 3rd ed; Pg 331
X-ray PNS
Proof puncture
Sinuscopy
Transillumination test
Nose and paranasal sinuses