Paul’s not just using a hyperbole to cover every possibility, but he is warning against a real danger, since we know that Satan and the fallen angels can appear to us as glorified angels from heaven. The real angels may never lie, but they do, and they can fool the undiscerning pretty easily.
Paul obviously knew NO angel from heaven would bring a false gospel...but one masquerading as an angel certainly could. Problem is: The human doesn't KNOW it's not an angel from heaven...unless they examine (test) the spirit...and the standard he uses as the test is: What do they say about the Gospel. This, of course, is consistent with 1 John 4:1.
Well, hey, Joseph Smith -- as advised by his Mormon god -- already told how to discern between "good" and "bad" angels:
Per Joseph Smith, you can "test" the nature of a spirit by the handshake ... oh, and by the color of his hair!!!
Smith claimed (Teachings of the Prophet Joseph Smith, pp. 214-215) to be able to "distinguish" between a "good" angel and a "bad" with this "spiritually offered" discernment:
"By the color of his hair; that is one of the signs that he can be known by..."
(Isn't it a "grand thing" to know that a demon can't deceive a Mormon thru dying his hair or something like that!)
Lds author Duane Crowther said: "These are the spirits for whom the handshake test of Doctrine and Covenants, section 129, applies." (Life Everlasting, Bookcraft, 1988, p. 247)
And what indeed does this D&C 129 section (which is Mormon "scripture") say about shaking hands to discern spirits?
D&C 129:1-9:
1 There are two kinds of beings in heaven, namely: Angels, who are resurrected personages, having bodies of flesh and bones—...
3 Secondly: the spirits of just men made perfect, they who are not resurrected, but inherit the same glory.
4 When a messenger comes saying he has a message from God, offer him your hand and request him to shake hands with you.
5 If he be an angel he will do so, and you will feel his hand.
6 If he be the spirit of a just man made perfect he will come in his glory; for that is the only way he can appear—
7 Ask him to shake hands with you, but he will not move, because it is contrary to the order of heaven for a just man to deceive; but he will still deliver his message.
8 If it be the devil as an angel of light, when you ask him to shake hands he will offer you his hand, and you will not feel anything; you may therefore detect him.
9 These are three grand keys whereby you may know whether any administration is from God.
So...THERE YOU ALL go! The "three grand keys" of discerning angels/spirits...per Joseph Smith!
Why, you don't even need to be a Mormon to practice this! (If only Muhammed had known about this test 1200 years prior to that...perhaps Islam would have never arisen!)
As you all perhaps can tell now, Smith had ZERO discernment on how to tell the difference between a demon masquerading as a ghost (Moroni), and a true angel.