In Chapter 18 of To Kill a Mockingbird, how can Jem tell which characters do not wash regularly?

The word "lavations" is just another word for repeated washing ('laver' meaning 'wash' in French).

Mayella's skin looked basically healthy and taken care of whereas her father's looked rather scalded, as if a fierce scrubbing to get him presentable for court ( and which he wasn't used to) had taken off the first layer of his skin.