That *looks* like a paint crack to me.

But to find out more, femove the fork, place it on some blocks - one under the dropouts and one under the steer tube - put your foot on the crown area near the brake mount, and slowly put put some weight on the fork and see what happens. It'd be nice to watch that area as you do so. If the fork flexes assymetrically - the "cracked" side flexes more - you have yourself a dead fork. Or any other indicator such as cracking noises or you can actually see a crack.

Don't put too much weight on it, or it will crack. But it will take a good amount of weight to do that - remember it has to withstand the front wheel hitting a pothole at 40 mph without failing.