I think we all agree that Edward would never come right out and tell Lucy that he was more interested in spending the rest of his life with Elinor Dashwood rather than with her. :-)

Elinor and Edward wouldn't be writing to one another, so there couldn't have been any letters for Lucy to "accidentally" find, during Edward's visit to Longstaple. ;o)

So as for "concrete" evidence, I doubt there was any of that sort. However, I believe that the incidents I've outlined in my previous posts are Lucy's concrete evidence. Yes, they're mostly suspicions, but as stated previously, they are enough for an intelligent girl such as Lucy. Her radar picked up on some concerns: Edward's changed demeanor toward her, the mention of Elinor Dashwood. With so much at stake, this would be enough to make her believe that Edward had feelings for Elinor.

My point is, that to Lucy, her suspicions are concrete evidence. She has a lot at stake. Edward, even with only two thousand pounds, is perhaps more than she'd ever thought she could get. As far as Lucy is concerned, Edward's changed demeanor toward her, and his speaking fondly of Elinor Dashwood, is enough to make her believe that her engagement is in jeopardy.

So Lucy takes action as though she does have concrete evidence. If she'd merely been fishing, I think she'd have left Barton once she'd gotten some proof. Even though Elinor was not about to confirm Lucy's suspicions, Sir John and Mrs. Jennings' teasing about Edward was all the confirmation Lucy would have needed, imo.