How can a doctor tell if you have a blocked sinus?

By History&Physical: The doctors diagnose any medical condition by first taking a detailed History and doing a Physical Exam and in some cases they order tests like blood test and/or Xrays,CAT Scan/MRI/Ultrasound and other tests In case of blocked sinus or sinusitis History and Physical may be enough but sometimes we have to do a CAT Scan to confirm our diagnosis. That is what doctors are trained for