One way, and it may seem trite.........check the Scriptures to see how an angel from God begins their conversation....
Every one I have found start out................ Fear Not .............. angels from God want us to know that they are not a threat to us......
I don't know of an instance when a demon appeared as an angel................... Scripture says the devil can, but does it also say his minions can as well? Dunno that........don't remember an instance in Scripture of that happening.........
Anyway, I go with the "fear not" theory.........
In the 2 instances we have of satan speaking to humans on Earth, He called God a liar, or omitted parts of God's Word.
1.Genesis 2:16 And the Lord God commanded the man, saying, “Of every tree of the garden you may freely eat; 17 but of the tree of the knowledge of good and evil you shall not eat, for in the day that you eat of it you[
f] shall surely die.”
Genesis 3:4 Then the serpent said to the woman, “You will not surely die. 5 For God knows that in the day you eat of it your eyes will be opened, and you will be like God, knowing good and evil.”
2. When satan tempted our Lord in the desert, he omitted parts of God's Word.
Matthew 4:
5 Then the devil took Him up into the holy city, set Him on the pinnacle of the temple, 6 and said to Him, “If You are the Son of God, throw Yourself down. For it is written:
‘He shall give His angels charge over you,’
and,
‘In
their hands they shall bear you up,
Lest you dash your foot against a stone.’ ”
Psalm 91:11-12 New King James Version (NKJV)
11 For He shall give His angels charge over you,
TO KEEP YOU IN ALL YOUR WAYS. Notice how satan omits this line.
12 In
their hands they shall [
a]bear you up,
Lest you [
b]dash your foot against a stone.
Our enemy will also take verses out of context to trip us up.
So we must test the spirits carefully by the the Word. If a handsome, smiling person, tells you what you want to hear, MUST be countered by checking the Word.