Case finding in RNTCP is based on –
"Sputum microscopy not only confirms the diagnosis, but also indicates the degree of infectivity and response to treatment" - Park
Remember

At least 10000 (104) bacilli should be present per ml of sputum for demonstration in direct smear on ZN staining
Culture can detect 10-100 bacilli per ml of sputum
Sputum culture
Sputum microscopy
X-ray chest
PCR antibody detection