Student or Learner
If someone tells me something and I wonder how they knew it, should I say
1. How did you know?
2. How did you know that?
Also, could you please give a few contexts and tell me which of the two questions is appropriate to ask?
Either question works. Including "that" (and stressing it) can imply "as opposed to something else?"
A: "I see that you took the bus from downtown."
B: "How did you know (that)?"
A: "You're still holding the ticket!"
I am not a teacher.