Question regarding Bardic Knowledge

[Greenfield]

I hate to say it, but it's looking more and more like yet another roll that the DM has to make in secret. The player whose character rolls badly shouldn't automatically know that the info they got is misleading.

 

[Dandu]

You could run it like Knowledge checks where you get information no matter what you roll, it's just the usefulness of the information goes up the better your result is.

 

[Ranes]

That's exactly how it works in Lost City of Barakus, where it can be used in one quest instead of Knowledge (Local History) or Gather Information, except for the more obscure information (DC25+), where only Bardic Knowledge and Gather Information get results.

 

[Tharkon]

I hate to say it, but it's looking more and more like yet another roll that the DM has to make in secret. The player whose character rolls badly shouldn't automatically know that the info they got is misleading.

Only the DC has to be secret, not the roll. Unless the player rolls a natural 1, but still, I prefer to assume players that play fair and don't cheat by using outgame information ingame.
If none of the player's characters has ever heard about a troll, then they don't know they need fire or acid, they will see that it won't be harmed by their attacks after trying but unless they happen to try fire or acid by accident they will likely have to retreat, or eventually knock the troll unconscious, if they are particular brace characters.
If they've heard a troll is weak to elemental damage, or even to cold damage, they should try at least a few shots using attacks of those types, if in the former case the first they try is fire (which would be successful), then they might still use a cold attack next, thinking it's weak against elemental energy. If they've heard most attacks don't harm it, but don't know it can still be knocked unconscious then they will likely run away after a single attack and find a library.