That's my general approach. They can see the d20, I know their defenses, so I tell them if it's a hit or miss. They have the opportunity to use the interrupt then before they know damage.
There are items later (in Dragon and/or AV) that have other similar effects but different wording. One item says "after you see the attack roll", some items have no wording (not even the "after you've been hit by...")... so it's hard to determine exactly what the rule should be, but in general, and especially since the power is already easily the best wizard utility 2, I don't have a problem with making the player think about it a little. Besides, shield does still help until the end of your next turn or something, so if there are other attacks coming your way, the shield might block them or divert them to a less menacing target like that juicy dwarf fighter over there...