correct usage

[JPS]

Which is correct, ' Do you know?' or 'Did you know?' I often hear native speakers using, 'Did you know?'

[Niall]

"Do you know?" is correct as a question.
"Did you know?" is more often used in speach. This question is also correct.

At first "Did you know?" seems to be using the wrong tense, but in fact it is simply asking "did you knew before now?".
When people ask "Did you know that London is in England?" they are asking "Did you know that London is in England before I told you?

If this is unclear, I will explain in a different way.
Hope This helps.

[Mordant]

Did you know? is often used when somebody has been the last person to find out and they are checking with everybody to see if they really are the last!

Do you know? would be used as in

Do you know where something is?

Do you know who has done this?

Do you know what's happening?