If herniated discs are present in 50% of asymptomatic population then how do you problem is really coming from spine esp if all clinical exam is norma?

Depends: Asymptomatic herniated spine can be exacerbated over time with signs ands symptoms of radiculopathy such as a tingling, numbness, and shooting pain. So it's really only a matter of time and degree of load to the spine which can eventually lead to symptoms.

Examination: Examination to determine if the herniated disc correlates with the symptoms is key. Sometimes electrodiagnostic studies are helpful.