I'd definitely base this on the cost of the catch.

+2 researchability?
It's generally safe to assume people just know unless there's some specific reason they wouldn't (such as not being clued-in at all).
For a PC attempting to research this level of catch, I'd call for a mediocre (or maybe average?) lore roll; even someone with no skill and a -4 on the dice shouldn't need more than a day to dig up the answer.  (Edit: Unless, of course, you can find a relevant aspect to compel; someone with Out-Of-Date-Knowledge might not have read Bram Stoker's Dracula, for example.)

+1 researchability?
It's generally safe to assume people don't know without a specific reason they would.  For example, of course the local Warden knows your catch; that's kinda part of their job and they've got access to the White Council's libraries.  If there's any question about whether someone would know or not, I'd tend to resolve it with a compel (for initial encounters), and just a quick check on "what sort of research resources does this person have?" for subsequent encounters.  For example, when Dresden first encounters the Gruffs, he has no idea that they're fae (and in fact is led away from that conclusion by their use of modern weaponry); I'd give him a fate point for that.  But then he just goes and asks Bob and presto, answer.
For a PC attempting to research such a catch, the first thing they'll need is an appropriate aspect invoke just to make the research possible.  Bob the Skull.  Access to the White Council records in Edinburgh.  Monoc Securities Contract.  The Merlin's Diaries.  No such aspect?  You don't even get to roll.  Once you do have such an aspect, you might still need a Good to Great lore roll - or perhaps substituting Contacts or some other skill appropriate to how you're getting the information.

+0 researchability?
These are the sorts of catches that tend to come up by accident more often than by design.  A plain wedding ring at an estate sale that burns without warning.  A fae with a perfect glamour to look like some other monster is attacked by someone with a steel knife - not because they know the knife will work, but because that's the only weapon the poor guy has.  An oriental demon, vulnerable only to the cleansing force of magical fire, finds itself up against one Harry Dresden.  I'd use aspects to cover this sort of catch almost exclusively, unless it was already well established that the catch was in play - for example, twin daemons only vulnerable to each other, wouldn't expect to get extra fate points when they fight - they know that going in.
For a PC attempting to research such a catch, the path is frustrating and fraught with false leads.  You find a relevant aspect, you make your research roll (as with a +1 catch), only you're up against Great or Superb or higher difficulty - and success tends to just tell you "The answer isn't here," unless you picked the exact right resource to ask.  (There's an excellent example of that in Cold Days, but I won't spoil it here; if you've read the book, you know what I'm talking about.)  It's probably easier to trick your opponent into revealing their catch (as a social take-out result) than to research it... if they even know what they're vulnerable to!

I'm pretty sure that's a houserule.  That said, I think it's a good (+3!) houserule, and one I'd be happy to use in games I ran.

The first two attacks are real.  The third is a sham, faking the death of an undercover agent who desperately needs to be extracted due to his acquisition of a valuable macguffin* the Baroness' enemies don't yet realize is missing.

* Possibly a non-physical macguffin; the true name of a particular entity or some other form of knowledge could fit well here.

^ What Taran said.  (I had a whole post typed out, but got sniped.  Oh well.)

Hah, no worries.  I just like dragons.  But werebears are pretty neat, too.

(I did, for the record, get to play an actual were-dragon character for a while.  Based it off a couple of quotes from various places - I don't remember if it's in the RPG books or the actual novels, but there's somewhere where Bob tells Harry "Hey, you know, you could do a really good wereform if you wanted to", and Harry's reaction is "But why would I want to?" - so I built up a character that was an ex-wizard who still had this one really good shapeshifting spell he'd worked out...)

Just for the record, I'd be curious to hear what you and your player end up working out about this.

Hm... I'd tend to say that's a good justification for a complete re-write of the high concept.  Perhaps something like "Demigod in training" or "Einherjar Runemage" (with a trouble of "But I'm not dead yet!")

Basically, though, it's looking to me like the rune mage isn't the focus of the character's powers anymore - they've (presumably) spent significantly more refresh on the physical upgrades from the mead.

Perhaps even something like "Freyja's Herald" (or Squire, or Emissary, or etc.), with the rune magic just implied as a "Well, Freyja's a norse deity, rune magic is quite thematic for an emissary".  In this case you could go with a trouble aspect of "...And Substitute Einherjar" if you wanted.

I'd probably run this as a reskin of the breath weapon power, actually.  Except that instead of breathing fire to attack / maneuver at range, the vampire momentarily mind-controls someone to take the action it wanted taken.  Let it use its discipline for such actions, with weapon rating based on what the subject has in hand, but also limit it to controlling people it can get an "eye contact" aspect on.

That way all of the PCs - even the one being "mind controlled" - still get all their normal actions, nobody's forced to stop playing, and it should be much faster & easier to adjudicate.