please give me some "how would you know .....?" example sentences?

How would you know who took your car, if you didn't see them?

How do you know what will happen in the future.

The first refers to how is it possible. Being possible

The second refers to the future. As in a hypothetical situation.

Q. How would I know if I've driven too far. 

A. If you pass the barn

Reaction to something impossible to know: " How would you know the solar system has 17 planets"?!. What I would say to just being told that. Given the general knowledge that there's 8. Also softens the blow, to me thinking the teller is insane.

If there are no windows in your room and no clocks, how would you know whether it is day or night?

How would you know where you are if no one speaks your language?

If oyu were the police, how would you know who stole the bike?

There is a common thread here, in case you missed it.  They are all conditional sentences, there is an "If" condition that you are asking about how you WOULD know.

How do you know that the class was postponed?

How do you know my name (I never told you)?

How do you know that the earth is round?

These are all question that ask how the knowledge you now possess came to be your knowledge, not if the knowledge exists and how you could come to possess it.

We use that expression when we don't know the answer and want someone to explain how it works. 'How do you know' can also be used. "How did you know' is only used if you already know the answer and want someone to confirm the reasons behind it.

How would you know what ingredients are in the cake?  (Read the label)

How would you know if someone likes you? (Look for smiles and happy eyes)

How would you know if your phone was charged? (Look at the battery indicator.)