I use Boxcryptor with plaintext filenames (to make it easier to recover previous versions from Dropbox).
This can be confusing (especially for less technical users I share folders with) because the two different views to that folder look just the same: if I go through Dropbox I get encrypted files, if I go through the Boxcryptor drive I see them as normal files.
But, as I said, there is no apparent difference until I click on a PDF, for example, and get a message saying it's corrupted. People can panic at messages like these...
Right now I use a text file called "Test Me.txt" that contains a few lines of text, and people can use it to confirm which view they're looking at: if the contents appear garbled, it's the encrypted view.
My suggestion is for you to study what is the best way to handle this doubt properly.
Perhaps an option to use plaintext filenames, but with a different extension: for example, a file called readme.txt could be saved encrypted as readme.txt.boxcrypted. You could register this .boxcrypted extension to show an explanatory error message when a user tries to open an encrypted file, and even let him proceed with opening after providing the password.
Thank you for a great program!