They used to come with a little certificate. I'll see if I can dig one out later and take a photo.
Before you buy any, do make sure you check the length / width to ensure that they'll fit your racquet. A lot of the ones you stumble across nowadays are 'single-handed' grips - an era when handles were shorter and no one hit a two-hander. These usually won't be long enough for a modern racquet. The difference is often in the width rather than the length.
My String Journey: http://tt.tennis-warehouse.com/showthread.php?t=362403