The only reason people even viewed the Asari as female is because they look like women with weird hair AND all seem to have sexy voices.
Liara regards herself as female (as does, I think, every single asari character, major or minor, that we meet over the course of the trilogy). Their government, their life stages - maiden, matron, matriarch - and their military - huntresses - are characterized by distinctly feminine nomenclature.
This seems like a very easy question to answer, and the only reason it was ever up for debate was because someone from Bioware said something stupid about gay romances. It's astonishing to me that it's still a thing.
They look different to everyone. Or they can.
As far as I'm aware, this is only ever invoked in that one weird "bachelor party scene," and I'd be very surprised if it's not something that's either actively retconned or simply just never acknowledged going forward. Asari are asari, and they look like asari.