vet du inte vad den är?

if we want to ask

vet du inte vad den är ?

can we ask like this?

don"t you know what that is?

or how can we ask?

If you say:

"Don't you know what that is?" it will become in Swedish " Vet du inte vad det är" and not "vet du inte vad den är" But i think that was what you meant to write. So yes, "Don't you know what that is?" is correct.

Hope this helped!!

Svara