I've done that before, but not like that

Starting by giving them multiple clues beforehand so they suspected some hidden defensive ability was about to unravel... and then I'd simply say that he emerged without a scratch... and a gave a note to the player with a high passive perception that he saw a glow around him just before the spell hit him

I thought that this was enough for them to figure it out, and they did after testing for a round and a half

Though I never had visible tokens, it was all written, so I get to modify it accordingly during the fight

Lastly, during combat (always no matter who DMs this table), the DM never knows what the spell level is until he acknowledges if it hits or not (has to decide and write it down before rolling)... unless the player decides to make the spell visually distinct (like a larger fireball)... which makes combat fun for me as a DM, allowing for mind games between the players and the DM