In Chapter 18 of "To Kill a Mockingbird," how can Scout tell which characters do not wash regularly?
As other answers indicate, Scout notices that those who keep clean on a regular basis have, well, normal looking skin, like Mayella Ewell. It is those who haven't washed in a long time, then suddenly try to clean up, who stand out. It must take a lot of scrubbing to remove the ground in layers of dirt, which then irritates and reddens the skin, as with Bob Ewell.
However, it seems that Harper Lee's point here goes beyond which characters keep themselves clean. That fact that at such a young age, Scout can pick up on the meaning behind such a minute detail shows that she has clearly inherited her father's analytical mind. It reminds us that we can believe this little girl's interpretation of the trial. Scout also seems to realize that failure to keep clean really has nothing to do with whether a person is poor or well-off, since two members of the same disadvantaged family have differing levels of hygiene. "Mayella looked as if she tried to keep clean, and [Scout] was reminded of the row of red geraniums in the Ewell yard." In spite of Mayella's lack of resources and her very rough home life, she does the best she can for herself. Lee reinforces this point when Mayella testifies about their home life, admitting that "the family hauled its water in buckets from a spring that ran out at one end of the dump . . . If you wanted to wash you hauled your own water." Through Scout's observations of Mayella's testimony, Lee reveals that it is every person for him or herself in the Ewell family, dominated by an abusive, drunken, father who clearly neglects his children and himself. It helps us to understand why Mayella would falsely accuse Tom Robinson, putting the man's life at risk, in order to do what is best for herself.
At the beginning of the chapter, Scout mentions that it was easy to tell which individuals bathed regularly throughout Maycomb, as opposed to yearly "lavations." A lavation is defined as the act or instance of washing. Scout goes on to mention that Mr. Ewell's skin looked scalded as if an overnight bath had removed years of protective layers of dirt from his body. She also comments that his skin looked sensitive to the elements. In contrast, Mayella appears as if she regularly attempts to stay clean. Scout does not go into detail, but one can assume that Mayella's skin was not scalded like her father's. While Mayella is on the witness stand, Atticus' line of questioning reveals Mayella's difficult home life. Mayella lives a rough life and is forced to raise her siblings because her father is an alcoholic. Despite living in poverty, Mayella tries her best to remain clean.
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.