Charlotte does seem to share a lot of information with her husband, who of course runs right to Lady C. with it. I'm sure Charlotte corresponds regularly with her family, probably her mother and/or Maria. They may have told Charlotte that Darcy has been accompanying Bingley to Longbourne. Since it isn't necessary for Darcy to be there -- there's nothing improper about a young man such as Bingley traveling alone -- Charlotte may have surmised that Darcy is going because he is courting Lizzy.