Not sure what you mean by "turned on", but $_FILES will always exist so long as you have an input type of file (and have a enc type of multipart/form-data of course).
By providing it with no file, than you can still find an offset in $_FILES under your input name. Since the very first thing you should do with the upload is determine if it was successful, you can check the $_FILES['value']['error']. Failing to fill select a file will result in an error code of UPLOAD_ERR_NO_FILE (the integer value of 4).
So to check no file in particular, you would use:
if ($_FILES['value']['error'] == UPLOAD_ERR_NO_FILE)
The only valid upload to allow you to continue is an error value of UPLOAD_ERR_OK or the integer 0.
You can find all the error codes for uploads here: http://php.ca/manual/en/features.file-upload.errors.php