According to the Main FAQ, all of them. But only the off-end.
Is a double weapon considered a two-handed weapon
a one-handed weapon plus a light weapon, for the purposes
of feats such as Weapon Finesse and Power Attack?
A double weapon can be wielded in two ways, either as a
one-handed weapon and a light weapon, or as a single twohanded
weapon. Either way, it follows the normal rules for
using such weapons.
Wielding as Two Weapons: The primary end (the “onehanded
weapon”) adds your Strength modifier to damage, and
the secondary end (the “light weapon”) adds one-half your
Strength bonus to damage. When using Power Attack, bonus
damage applies only to the primary end. If you have Weapon
Finesse, you may add your Dexterity modifier instead of your
Strength modifier to attack rolls with the secondary end (but
not with the primary end).
Wielding as One Weapon: Add one-and-one-half your
Strength bonus to damage rolls. When using Power Attack,
double the bonus damage granted by the feat.
The Sage strongly recommends that characters who wield
double weapons record attack and damage values for both
methods of use, to speed play.
For what it's worth.
Note that this answer is supported by the Core rules, as long as one assumes that the text of the Power Attack feat that states "Normally, you treat a double weapon as a one-handed weapon and a light weapon" to be globally applicable. Since this assumption causes fewer headaches than assuming it is only relevant to the Power Attack feat, it's the one I favour... so I agree with the Sage here that the off-end can be finessed.
-Hyp.