If it's a text (ASCII) file, there is no way to tell if the file is completely transferred unless the customer provides a separate checksum file of some sort (i.e. a file specifying the file's size, contents, etc.) or if you can determine completeness based on the file contents (e.g., the last line of the file contains "EOF" or something). You can test for the file's existence, but a partially-transferred file would test "true" for existing, which doesn't help.