It is not a separate file - it is information embedded in the JPG image.
If you are using Firefox, there is an extension that will let you check information with a right click. In Windows (Vista only?) you can apparently check under properties (right click menu). In Linux, you can see EXIF data with a program like gThumb or, on the command line, with exiv2 and other programs.
Or, keeping things simple, if your picture is on Flickr, just check you have enabled the setting on your account that enables other people to view more information about your shots.
Lots of options!
You could of course just add a note of when the picture was taken - since there is no money or other prizes riding on this "competition" we will be trusting people to be honest
Wulf