The phrase: How can you tell?

What does the phare ‘how can you tell’ mean in this conversation:
A: I thought he was a liar.
B: No, He’s a good guy.
A: Well, how can you tell?

Please help me, thank you?

It means “why”

It means “why”.

It doesn’t mean ‘why?’
It means ‘How do you know that he is a liar?’

thank you Beeesneees, I get it!

thank you!Dear,my professor.