It depends on the file type you are uploading and any restrictions you are imposing. Look up the following PHP methods for more information:
is_uploaded_file
move_uploaded_file
mime_content_type (if you only accept gif, you want to catch and fail on bmp right?)
getimagesize (to check upload size limits if using images)
filesize (if not an image)
Between these, and basic checks, you should be able to determine validity of your upload.