How do we know they're Inhumans? Well, there are a couple of puzzle pieces to figuring this out, but it is not a stretch of the imagination, in fact, I'd bet on it.
Marvel has released it's teaser images for the new Inhuman comics series:
And apparently the comic storyline is about mad people being exposed to whatever it is that creates Inhumans (I'm not an Inhuman expert). So it looks like Inhumans are the new mutants in the Marvel comic world.
So why use Inhumans instead of Mutants? Why have readers familiarize themselves with Inhumans instead of Mutants when judging by these posters, the premise looks pretty similar?
1) We know that Inhumans is a movie franchise that Marvel wants to work on.
2) So far in the MCU, anyone with powers is a god (Thor) or is connected to the Super Solider Serum (Cap, Hulk, Red Skull). Marvel knows they can't keep milking that cow.
3) They are NOT allowed to reference mutants in the MCU
So how can they explain where these people with powers keep coming from? Including SW and QS?
The answer is that SW and QS, and many future Marvel movies characters, will be Inhuman. This will also disconnect SW & QS from Magneto as their father which must be done. On top of that, this will lead into an Inhumans movie and set up an Inhumans universe quite nicely, as well as many other franchises (The Wasp in Ant-Man, etc.)
Thanks for reading! Feel free to talk shit about me in the usual place.