But if I wind up with the real thing, is it still "not acceptable?"
Wouldn't it be wise to examine what you have obtained and personally discern whether or not you have the real thing ?
Of course. No one is talking blind allegiance here.
It may, very well, turn out that what your friend has given you is valid, but it never hurts to check with the final authority.
You have an infallible way of doing this?
SD
God, the ultimate author and finisher of our faith, entreats us to ask Him to provide us with His wisdom (i.e. His truth), and promises to honor such requests by (infallibly) providing such wisdom to us.James 1:5 If any of you lack wisdom, let him ask of God, that giveth to all men liberally, and upbraideth not; and it shall be given him.