Question: How can I know if I get into the "astral" plane during meditation?

Answer: The material plane is visible from the "astral" plane. Some magicians even develop the ability to actively influence objects of the material plane with their "astral" body. One can also learn to condense one's "astral" body (a "double") to the state that it is not distinct from an ordinary physical body in appearance. It is this that is so attractive to mystics who do not know God and who strive not towards Him, but towards acquiring siddhis (super abilities).