In Logan, the titular Logan along with Professor X and Laura are traveling when they stop to help the Munsons, a family of farmers. Throughout the encounter, Logan and the others keep their true identity secret. The family offers them food and shelter and during dinner, it's clear that Laura is not normal as she eats with her hands at first and does not talk.
I figured that the family would have just assumed that Laura was developmentally challenged, especially since Charles Xavier (ostensibly Laura's grandfather) talked about running a special needs school. But apparently the father knew from her behavior that what she was a mutant since he asked Logan:
How long has she been a mutant?
Since Laura didn't exhibit any of her mutant powers, what made him think that she was indeed a mutant?