Well, one way to *know* for sure that you're having psychic impressions is to get verification for the information you're picking up directly from the person you're reading for.
I'd suggest taking the advice of the other poster and use this board as a practice ground for verifying that your impressions are legitimate. I'm sure there are people here who would be willing to let you read for them and ask them if the stuff you are picking up is correct or not, and give you clear feedback on the material you receive.
If I can be of any further assistance, just ask
Best wishes,
--Meri