1. Agatha thought her son's soul was in teen's body.
2. Rio told her it's not her son.
3. Teen speaking the name "Nicholas Scratch" off the Quija board confirmed the sigil isn't protecting the name "Nicholas Scratch"
This allowed Agatha to realize teen is not her son.
4. Teen confronting her about what being a witch is about gave her Wanda Vibes.
5. Teen is ultra surprised everyone is super callous and all about power, Wanda vibes.
6. Agatha said sigil's are for amateurs, Agatha considered Wanda a amateur, powerful but an amateur so off course she would use a sigil.
It just all came together in Agatha's head when teen confronted her about power.
7. Someone had to put the sigil on teen, it's prolly Wanda.
8. When Agatha told Wanda, "You're going to need me someday." Wanda said, " ill know where to find you."
My theory is when Wanda was crushed she exploded into energy and got stuck on the witches road.
She has somehow been able to communicate with teen (Billy) to get Agatha to get to the Witches Road to save her (Wanda).
Also when Agatha says "Are you sure" to Wiccan saying "I dont want to go on the road anymore" she knows he's lying cause he is there for his mom.
I also assume teen is fused with the soul of the hex Billy, when he died, his soul/energy went into this kid's body. The soul of "Billy Maximoff" went into the body of "Billy Kaplan"