Load the GECK, open a file, then look at the object window.
The tree should look like this on the left:
Creature
Leveled Creature
Leveled Character
NPC
Open up NPC, then double click on a name in the right section.
In the popup, there's a row of tabs at the top. They say:
Traits Stats Factions AIData [etc]
At the far right of these tabs are arrows. Click repeatedly on the arrow that points right, until you see the tab that says "Head Parts". That should be what you want.