@candle @Hiraelle I'm pulling that out of my ass, but I think that it's "le/la" when it's a transitive verb, "lui" when it's intransitive.
So, "I see him" => "je le vois"
"I give to him" => "je lui donne"
That's the only occasion where le/la can be pronoun instead of a determinant, though. And since it will always be right before the verb of the sentence, you can't mistake it for the usual determinant - which is probably why it's used to mark the difference transitive/intransitive, by the way. :<