Hi,
First do your upload in the AT SELECTION-SCREEN event. Then the logic would be read the file or rather upload the file with GUI_UPLOAD then.
Read the first line with READ and INDEX. Then Split based on the separator say into a work area that has 30 fields each say text of length 20 chars (or what ever is max length this is just to validate). Then check the 7th field in the work area if it is blank its the file with 6 fields and if not its the file with 30. Give you message and as its AT SELECTION SCREEN user gets it on the screen
Cheers,
Arindam