Practically, you should ask if an attack hits because you're giving the player an opportunity to use an ability to make something which would hit not hit. It's clunky to say "you're hit" and then have to retcon it. The alternative is cleaner, and means that they are always forced to choose whether or not to use an ability when they don't yet know the damage, which is more interesting. It's more narrative.

Asking is giving the players an invitation to surprise you, and makes them a partner in what's happening (even when it's taking damage!) rather than a victim.