As other monks have pointed out, the commands supported by the ftp server daemon are the determining factor. You'll need to test based on individual cases. It is not possible to build a completely portable solution if you go beyond the most basic ftp commands.

You might try using ftp->size(). On servers that support this command, it will return a size in bytes for the file listed (and a 2NN status code), or return an error code (550 Not a plain file). This works on my UNIX/Linux servers.