The transfiguration isn't the same and belongs in a unique category as a one-time thing. Even after his resurrection, Jesus seems to have appeared in normal human form except for his wounds. I don't know exactly what Paul saw in his vision other than blinding light.
The scriptures do talk about spirits and testing them. The test as described in the scriptures has never been applied to any of the apparitions I've studied. The spirit is supposed to affirm that Jesus is the Christ the son of the Living God. There could be other spirits named Jesus.
"The transfiguration isn't the same"
Hey, deceased saints appeared to living men. That is "same" enough for me.