So the following situation came up in a recent session: A party of PCs (built at chest deep) ran into some out of control red court vampires (in horrible bat-monster form, no flesh masks). Our wizard decided to create a magical block against them, then the player said, "Hey, I'll make it a spirit block, making my pentagram (a holy symbol to my character) shine with bright light, to take advantage of the vampire's vulnerability to sunlight and holy stuff. Wait, does my character even know about that weakness? Do I need to roll lore first?"
Possibly. I'd rule that anyone with Lore will know about the catches of anything supernatural you'd rate as commonly occurring within the supernatural world, which would certainly include Red/Black/White court vampires and Fae.
As for obscurity, anyone with Lore is going to know a +2 awareness catch without needing to roll. A +0 catch can only be discovered through play, not necessarily in combat though, tracking down a former associate of the big bad to find out how to kill it might work too. +1 catches are where the rolls would come in, I'd be generous to the monster; maybe making the difficulty equal to the sum of it's two or three highest skills or power costs (two for a general type of critter, three for extremely rare or unique individuals). Per the Answers trapping under scholarship, if the Lore roll fails (and it should fail pretty big against things like major demons), it moves to the Research trapping, where shifts of time can make up the difference. The reason I went with skill or power rather than a flat number is that more dangerous creatures are more likely to kill those who oppose them, leaving a very small pool of people who figured out their catch and survived to pass along the knowledge.
Of course, that assumes that the character in question even knows that kind of critter they are looking at. A Lore check vs the target's highest skill or power cost would be reasonable to determine if they even know what it is; with a failure possibly misidentifying it as a similar creature.