there's a multiverse. And anything can happen within it and this story starts with a baby who was orphaned, lost her twin, and gained a new family. This said baby was Wanda Maximoff, or rather Wanda Parker. She was taken in by the Parkers after being abandoned if Latveria and now loves with Peter Parker and their aunt May in New York.
Wanda discovered she had this chaos magic within her and Peter, like in most stories, is Spider-Man. Wanda was introduced to Dr Strange, a distant man who knew magic and soon became his Apprentice, now called the Scarlet Witch.
Stephen Strange helps her a lot but there's one question she still needs answering, is that it she's a mutant or no. she wants an explanation for her powers. So Dr Strange teleported her and Spidey to Xavier's School for Gifted Youngsters, aka the safe place for all mutant kind.
And the brain of the operation is Charles Xavier, one of the strongest telepaths in the world and with the help of Cerebro he can locate any mutant, so he used Cerebro to find out if Wanda is a mutant or not.