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}]}`