Sometimes it can be difficult to tell even when I have physical access to the device. To me it looks like it may just be a scratch. I rarely see the glass crack on the Retina laptops. Usually when the screen cracks on a Retina laptop, the LCD Panel is what cracks and will easily show up in various ways that leave no doubt there is damage (surprising the glass rarely cracks when the LCD Panel is cracked). Only someone with physical access to the laptop can tell you for sure.
There are ways to test, but I cannot post them here as the moderators would remove the post. It really doesn't matter anyway if you can still use the laptop without problems, but if the mark interferes with using the laptop, then you can get an estimate for repair (it will be expensive).