It's hard to say for certain, but 4e tends to be pretty generous about such things. For instance, when a Staff wizard uses his defensive Immediate Interrupt, he can know everything about the attack results, including the damage roll, before deciding whether to avert the attack.
I'd suggest erring on the side of generosity, and allowing the player to be certain the power will have a chance to make a difference before they expend it. Especially if it's an Encounter or Daily power.