Yeah, this is the way.

We just fought a Troll in a Pathfinder session I was in. I'm playing an Athamaru (fish person) new to dry land, so I don't have a ton of knowledge about stuff like fire. But the Druid hitting it with a fire spell, and the GM describing the way the Troll reacts is enough to naturally gain that knowledge on the spot. There are all kinds of reasons a character might not know even common monster weaknesses.

I think doing this kind of metagaming is important, because it gives opportunities for specific characters to stand out. If you have a party member with monster knowledge, it's cooler for them to yell a warning, than it is for everyone to just act like they already know