since we don't know its mass,, is it by looking at the lepton by products?
When measuring a [itex]\nu[/itex], we don't have the sensitivity required to idendify the mass eigenstate.
we measure the flavor, by identifying the flavor of the lepton produced.
See for example opera's measurement of tau neutrinos in a muon neutrino dominated beam
by identifying the tau decay http://arxiv.org/pdf/1006.1623v1.pdf
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