You will know if it is your third eye when it happens both with eyes closed and eyes open. Third eye is not operating with the physical optic system.
But they are subtle eyes in the subtle body which is used in lucid dreams and astral projections. these subtle eyes may operate with or without the physical eyes. More than likely you are experiencing their vision while seeing physically at the same time.
One more note: third eye is always a singular vision. It operates through one vision opening. Eyes of the subtle body are dual vision just like eyes of the physical body.
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See my book Meditation Pictorial to get some ideas about this:
https://www.createspace.com/Customer....do?id=3380228 Amazon.com: Meditation Pictorial (9780981933221): Michael Beloved: Books
UK source:
Meditation Pictorial: ISBN 098193322X : Michael Beloved : EAN 9780981933221 : Book