Caroline must have an idea for the growing admiration that Mr. Darcy and Elizabeth have been showing. I think she has for sometime known that Darcy fancy's Elizabeth but it isn't until now that she has any real fear of either party acting on those feelings. Caroline has always had a way to put the Bennets down when ever she has a chance and uses her opinion to gage how Darcy is feeling. If when she had said

"I must confess that I could never see any beauty in her."

and continues to pull apart Lizzie, she is doing so to try and see how Darcy is feeling but also because she is angry of his regaurd. The narrator says following such statement.

"Persueded as Miss Bingley was that Darcy admired Elizabeth, this was not the best method of recommending herself; but angry people are not always wise"