Use the Knowledge Skill as a guide.
I used the rare knowledge(nature) check (15+CR) DC 19 for general info on Serpent Folk and a DC 20 Knowledge(History) unless it was stated that it was higher in any place.
I say until the end of "Souls" they probably shouldn't even get a check to see if anyone even knows what they are.
Generally speaking, since serpentfolk are CR 4, a DC 14 Knowledge (nature) check is enough to see one and go "Yep, that's a serpentfolk, and they're monstrous humanoids." At each +5 DC beyond that, (DC 19, DC 24, DC 29), you gain additional information, which various folks handle various ways. As written, at each tier of knowledge, the character learns one new fact (a special attack, or a defensive ability, etc.) I'm more generous myself and tend to offer an offensive quality, a defensive (or otherwise notable) quality, and a bit of "ecology fluff," such as languages, habits and so on.
I sometimes also ponder adding a +5 modifier to the DC if a creature is particularly obscure (such as a creature only found in the arctic in a tropical campaign, or one thought long extinct), or possibly even a +10 if it's incredibly obscure (native to another world, or vanishingly rare), but I haven't actually ever put those house rules in place. If I did, I'd go with the +5 modifier for the mostly forgotten serpentfolk.
I also write up lore tables in advance*, going from "most essential" to "most obscure" in order of revelation, but other GMs use entirely different means of determining what the PCs learn. I think James Jacobs asks his players if they want to know something offensive, defensive, or otherwise about the creature, and then selects something from that category.
* Despite being in the early stages of running Serpent's Skull, I haven't actually typed up a lore table for serpentfolk yet (and don't have time at the moment to do so on the spot), or I'd include it here.