The family could tell that Judith was in love because her mannerisms changed and she seemed happier.
When Judith is in love with John Holbrook, the family is a little surprised at first. His attentions (and intentions) were never obvious. One thing that the family is sure of is that Judith is in love. Her entire manner shows it.
Not even her father could have failed to guess that Judith was in love. She had never spoken another word, even to Mercy or Kit, after that first surprising disclosure. But there was a brilliance in her eyes, a warm color in her cheeks, and a new sweetness in her manner. (Ch. 10)
There is no shortage of suitable men, it would seem, and John Holbrook is poor and wants to be a student. He may not have thought that anyone was actively considering him. He does spend a lot of time with the Woods, but never says anything specific to Judith. John never “formally asked permission to call” but came when Rachel invited him, and kept coming. The family assumed he wanted Judith.
There had never been the slightest hint that he was courting Judith. He never seemed to single her out, but sometimes he consented when she suggested that they walk along the green in the twilight. That was all the encouragement Judith needed. Indeed, it was more than enough to satisfy the whole family of John's intentions. (Ch. 10)
John becomes part of the family, and they all enjoy his company. He reads aloud and the family gathers around, and it sometimes also makes the uncomfortable silences between Kit and William more comfortable. Kit considers him a friend.
Kit realizes one day, while they are sitting around listening to one of these readings, that Mercy is in love with John Holbrook too! She looks up from her knitting and notices something odd about the expression on Mercy's face.
Mercy sat, as usual, slightly in the shadow beside the hearth, her needles moving so automatically that she rarely glanced at her work. … Those great listening eyes were fastened on the face of the young man bent over his book, and for one instant Mercy's whole heart was revealed. Mercy was in love with John Holbrook. (Ch. 11)
So just to recap, both Judith and Mercy are in love with the same man, and Kit is not really interested in the man who is courting her. No one really knows who John is in love with.
Despite this conundrum, it all works out in the end. Kit does not really want to marry William Ashby. He is not the right fit for her. Judith, however, was enamored of him before switching her attentions to John Holbrook when William began courting Kit. Kit is more interested in Nat, and John is more interested in Mercy, so in the end it all works out for the best. Everyone is paired off, and everyone is happy.
It may seem as if there is a lot of switching and settling of love here, but you have to remember the time period and the situation. Kit, when she first tried to compromise with William, thought she was doing what was best for her future. However, William Ashby was Wethersfield all of the way, and Kit was like a tropical bird, as Nat always knew, and did not quite fit in. In the end, Kit’s coming to her senses is a natural thing. William and Judith also both want the same things, and Mercy and John Holbrook are both well-matched in temperament.
Love in Colonial times must not have been an easy thing. Conditions were harsh, and the future uncertain. Yet even within this, life goes on. Young people fall in and out of love. Sometimes it takes some time to figure out who you really love, and who loves you. In this situation, you want someone who also wants the same things. Everyone in this book does seem to eventually find that.