I don't think you'd know unless the deity in question chose to make it so. Even so, deities often make their will or intention known through subtle signs. Could someone become a paladin this way? I think it's plausible, if the appropriate deity notices them and decides they need a push, but it wouldn't be a formal sort of paladin as we think of them (Order of the Radiant Heart, etc) and if the deity didn't make themselves and their will known, I'd suspect it would be a temporary empowerment as long as their goals aligned with behavior the deity in question approved of. (I mean, you think. I would imagine that they'd make their involvement known eventually though, perhaps after watching for a while to see if the person in question would make a worthy divine champion.
