She has an arm, which contains pressurized blood.

"We could probably check by seeing if things I know from my maybe-mistake memories are things that I would not - probably - know in other ways. I know how to operate a space truck but if that's - not... the way things aren't... - if I'm an alien my world's space trucks will operate differently than yours. I know how to maintain my truck's reactor, I suppose I could at least run the design by a nuclear engineer for - how much they think it will work when they think about it - Also there's this expressive language problem I keep running into."

"Yeah, I put on your charts that your speech is disfluent, which is a bad sign with a head injury. I'm considering it somewhat likely you've had a minor stroke, but the scans'll catch that, even if I'm pretty sure they can't tell the difference between missing memories and being a time traveller or an alien."

"As far as I can tell my speech is perfectly fluent unless I'm talking about numbers or the thing I can't remember any of the words for! A stroke does seem pretty likely though and it's fair to check that before calling in a nuclear engineer."

They send her off for scans. 

"Well, you're not having a stroke, which is good news for you because strokes are dangerous, but bad news for me because now I have no idea what to put in your chart. You really think we should call, uh, a nuclear engineer?"

"It's a way to test if the things I know from my maybe-mistake memories are things that are not mistakes! Unless nuclear engineering is very simple and everyone learns it in school. In the world I remember nuclear engineering is not very basic and people mostly only learn it if they have relevant jobs."

"I sure don't know any nuclear engineering. I'll call a nuclear engineer for you. I'm not actually sure if I'm supposed to do that but I hate having a patient who's a complete mystery, and it's weird enough I don't think I'll get fired for it."

Off the nurse goes. 

The nuclear physicist shows up about an hour later. "Hey, uh, your nurse said you have the kind of medical emergency that requires a nuclear physicist and might involve time travel and I would love to help. Though it seems pretty unlikely time travel was involved."

"Either I have a lot of memories that don't correspond to things that happened outside of my head, or I am an alien, or I am a time traveller, or something else is happening that I haven't thought of. Some of my memories that might be mistaken are about maintaining a small molten thorium salt reactor. If my description of how a small molten thorium salt reactor works is accurate, and there aren't any nuclear engineers matching my description who have recently gone missing, which I assume there are not or someone would have identified me already, then my memories are most likely not mistaken. If, better yet, you can confirm that my reactor design would work but is not in use anywhere in the world then I am likely an alien or time traveller."

She says this kind of slowly because all of this does not seem to have been obvious to the people around her, and if she's from another universe she's pretty sure, based on the gaping holes in her new vocabulary, that she's been transported to one where people are much stupider and haven't invented the concepts of [truth values] or [inference]. She would've bet a lot of money yesterday that those things came earlier on the tech tree than, say, nuclear reactors but apparently not.

(OK, actually it's much more likely she's just having a bizarre fever dream, in which case none of these people or the world around her is real at all, but there's no point in operating in that world because she has already tried everything she can think of short of suicide to wake herself.)

" - that was a very intimidating speech from a likely delirium patient," he says. "I am nervous and slightly aroused. We do not have small molten thorium salt reactors, because most people don't like nuclear energy, because radiation is invisible and scares people. We do have papers about them so you could have just read one of the papers and then suffered - some kind of bizarre horrifying brain damage, if the design you know is the one in the papers."

How about she just explains how the reactor works and ignores the sexual commentary from this random physicist who JUST MET HER FIVE MINUTES AGO.

He takes notes. "I'm going to need to - compare this with the papers we've got, this isn't my exact specialty. But. There are not a lot of people who have that information and I don't think any of them mysteriously went missing, and you are very coherent and good at math for someone with a brain injury. Or even someone without a brain injury. Also cute. I'm leaving now."

That was weird.

The nurse comes back in. "All your scans are normal. I have to decide whether to refer you to psychiatric or discharge you as healthy. Any idea whether you're crazy?"

"I have the belief that I'm an alien or a time traveller, a noticeable vocabulary gap, and I express numbers in a nonstandard way. I am maybe delusional but I think not in a way where I am a danger to others."

"If I discharge you are you likely to end up arrested for shoplifting or home invasion or something in that class where I'll get hassled over having decided to discharge?"

"I do not have any known criminal tendencies and don't expect to commit any crimes that I can anticipate being crimes, but all my memories are from a different world and if this country criminalizes stupid things I may break those laws in ignorance."

"This country criminalizes a couple of stupid things but ignorance is a defense and they're not the kind of things where someone'd haul me up to demand to know why I released you. Do you want psych or discharge."

"DIscharge, please, and directions to somewhere where I can get oriented in this city?"

"Sure. You're at Annapolis Central Hospital; you probably want to go to City Hall for new documents? You can explain you're not sure if you're an alien or a time traveller or have amnesia, you'll need documents regardless. It's four blocks to Washington and then left, and then you'll know City Hall when you see it. I don't know what they'll do about housing you, but it's not my job and it is theirs so probably they'll figure it out. If you're actually an alien or a time-traveller then they'll want you to go to Central Island Agency, of course, but I think that's not exactly known for sure yet? I don't know if they''ll want you to go there when they don't know for sure."

"Thank you!" She will go to City Hall for documents.

City Hall's a weirdly short Greek-inspired building that seems to have several wings: 'sentencing', 'documents', and 'mayor'. Waiting in line in Documents is an elderly woman ("the dog ate them", she says morosely), a cheerful set of newlyweds, and a grouchy set of newlyweds.

"I'm either an amnesiac or an alien, and I need some documents! And I guess a way to earn enough money to rent an apartment?"

" - wow. Uh, now I'm not sure whether to follow the procedures for amnesiacs or the procedures for aliens but I'm leaning towards the procedure for amnesiacs because I'm less likely to be laughed at."

"Do those come up ever?"