My girlfriend had lip piercings (snakebites) for a long time and took them out a few months ago.
You can definitely still see the scars>>>>http://i30.photobucket.com/albums/c317/violetskies143/scar-1.jpg
However, this is probably because she gauged the holes to a size 10 instead of the standard piercing size.
I know other people who did not gauge their lip piercings and you can't see the hole.
It would depend if there were any incidents with the piercing as well (such as infection or if the ring got pulled out, etc.)