The good news: In the current production release, there's a method called DirectoryExists in FTPConnection, but it's internal. If you change it to public and recompile the project then you'll be able to use it. It will be public in the next release.
The bad news: All it does is try to change into the directory, which is the best generic solution that we know of. We have a policy of not including server-specific workarounds, so that's the best we can do.
- Hans (EnterpriseDT)