Speaking in Edinburgh on Friday (January 31) - the day Britain leaves the European Union - Sturgeon said the question of whether the Scottish Parliament had the power to agree to hold a non-binding vote on independence had never been tested in court.
Sturgeon wants to hold another Scottish referendum, but she cannot do so without the consent of the British government.
She has asked Johnson to enter negotiations on transferring power to hold a referendum from London to Edinburgh, and said such a step was the best way to put the legality of a vote beyond doubt.
She said a non-binding referendum could gauge the appetite for an independent Scotland.
But it was uncertain if one could be held under the Scottish government's existing powers, she said, and a legal test could take the cause forward or equally set it back.