One who perceives what is not yet evident. A seer reports what they see; seeing is not a claim of certainty.
An agent noun built on see (Old English seon, to see, from Proto-Germanic *sehwan) with the suffix -er; a seer is named for the seeing, not for any certainty attached to it.
One who reports what they perceive before it is otherwise evident — the claim is only ever what was seen, never a certainty beyond that.