ref chilepepper

Actually, that's incorrect.

SRD
Concealment Miss Chance

Concealment gives the subject of a successful attack a 20% chance that the attacker missed because of the concealment. If the attacker hits, the defender must make a miss chance percentile roll to avoid being struck. Multiple concealment conditions do not stack.
If you want to keep this all super secret, the attack chooses the square and attacks. A ref will verify if it's a hit and ask the defender to roll the miss chance. If the attacker doesn't get to know the roll result, but if he can otherwise see the projectile hit or miss, he'll know. The defender would see the projectile whiz, or get hit by it depending on the roll.