The Inventory tag doesn't work on armor stands, but the Equipment tag does.
/testfor @e[name=Skeletor,Equipment:[{id:"",Count:,tag:{}...},{id:,Count:...},{id:...},{id:},{id:}]].
The five arguments come in this order: Held Item, Boots, Pants, Chestplate, Helmet. You want to test for boots, so I would try this command:
/testfor @e[name=Skeletor] {Equipment:[{},{},{},{},{id:golden_helmet}]}