I think Elinor is sure Edward does not love Lucy because he doesn't show that love. Elinor's smart enough to figure out that Lucy told her about the secret engagement in the first place because Lucy thinks he loves Elinor, not her.
Lucy makes much of the letters he sends so faithfully, but Elinor would know that his excuses for not visiting Lucy more are just that -- excuses. Because of how strictly stratified that society was, Edward could have visited Lucy while she was staying with her merchant relatives while running very little risk of discovery; even if someone should happen to see him there, he could explain it off as a duty call to the children of his old tutor.
A man in love likes to be near his love; Edward does his duty by maintaining the relationship through letters, but he avoids actually seeing Lucy. So long before Edward fesses up, it's clear to Elinor he doesn't love Lucy, and hasn't for some time.
She also knows Lucy, who has a great deal of personal charm, is intelligent and skilled at shaping her behavior to the environment (compare her use of language to her sister's), and does whatever necessary to get her way. Add in the fact that Edward tries to think the best of people, and it's pretty easy to imagine Edward being initially attracted and her bringing him to the point of proposing before he realized how shallow she is.
Lucy makes much of the letters he sends so faithfully, but Elinor would know that his excuses for not visiting Lucy more are just that -- excuses. Because of how strictly stratified that society was, Edward could have visited Lucy while she was staying with her merchant relatives while running very little risk of discovery; even if someone should happen to see him there, he could explain it off as a duty call to the children of his old tutor.
A man in love likes to be near his love; Edward does his duty by maintaining the relationship through letters, but he avoids actually seeing Lucy. So long before Edward fesses up, it's clear to Elinor he doesn't love Lucy, and hasn't for some time.
She also knows Lucy, who has a great deal of personal charm, is intelligent and skilled at shaping her behavior to the environment (compare her use of language to her sister's), and does whatever necessary to get her way. Add in the fact that Edward tries to think the best of people, and it's pretty easy to imagine Edward being initially attracted and her bringing him to the point of proposing before he realized how shallow she is.