Again, the writer of Samuel says it was Samuel.
1 Samuel 28:12
And when the woman saw
SAMUEL, she cried with a loud voice: and the woman spake to Saul, saying, Why hast thou deceived me? for thou art Saul.
Not one manuscript in Hebrew, Aramaic, or Greek says anything different.
The woman saw Samuel, not an angel, not a demon, certainly not a lying spirit, because Samuel tells Saul that Saul and his sons will die in battle. Which is what happened.
But here is the kicker, and it is the narration of the writer of 1st Samuel.
1 Samuel 28:15
And
SAMUEL said to Saul, Why hast thou disquieted me, to bring me up? And Saul answered, I am sore distressed; for the Philistines make war against me, and God is departed from me, and answereth me no more, neither by prophets, nor by dreams: therefore I have called thee, that thou mayest make known unto me what I shall do.
We are told that this was Samuel.
We are told this by the narrator of the story.