Proof

Independent witnesses in whose presence dowry was demanded. Exact conversation can be recorded where demand for the dowry was made; such recording is admissible in the court provided the authenticity of the recording is proved.

In fact, dowry has been demanded from the girl when she was alone. she does not have any recording or any witness. But  she told  the incident  to one of her relatives when that incident happened. How to prove ?

Totally in hands of lawyer, nothing can be said in concrete.  If you have taken dowry give it back, its worse than begging, if you have given dowry, then you should not have as it is also punishable crime upto 7 years jail.

These matters do not do good to anyone.  It is simply waste of time, in your case if there is no evidence like Mr Vijay said, dowry demand cant be proved, be ready to face perjury for lying under oath.  For that also jail 7 years.

Best is to settle amicably.

who will demand dowry in front of independent witness?

But peculiar  behaviour of in-laws can be proved.