Can a nasal smear be tested for leprosy?

Yes. Biopsy of the involved skin is a better test. The biopsy needs to be examined by a pathologist and usually requries special stains.
Yes. Leprosy can be detected by searching for the acid- fast bacilli from either nasal smears or skin biopsies. Molecular probes & serological assays (phenolic glycolipid-1 or lipoarabinomannan) are also used.