Uller
Adventurer
You cannot target something that has total cover. If you try to place the point of origin at a location that is behind total cover, the point of origin becomes the near side of the total cover. A Wall of Force provides total cover. You can't target things inside it. If you try to place a Fire Elemental on the other side of a WoF it appears on your side.
From the SRD under the Targets section of Casting a Spell:
To target something, you must have a clear path to it, so it can’t be behind total cover. If you place an area of effect at a point that you can’t see and an obstruction, such as a wall, is between you and that point, the point of origin comes into being on the near side of that obstruction.
. Fireball can't go through it, becuase it requires a streak to physically pass through the WoF. I was just contrasting to a physical, opaque wall that would stop a spell like sacred flame due to line of sight. Since it doesn't move from the caster to the target, I'd rule the WoF has no effect on it. But you're right, Sage Advice might further clarify things either way.