Try this command after you spawn a skeleton:
/testfor @e[type=Skeleton] {Equipment:[{id:bow}]}
It will then say something like: Skeleton did not match datatag.
Even though it has a bow in it's hand.
Comments 6
FVBico, your command doesn't work either. The 'minecraft:' isn't required in give commands, but when using testfor, the json must match the entity's NBT data exactly.
Try something like this: /testfor @e[type=Skeleton] {Equipment:[0:{id:"minecraft:bow",Damage:0s,Count:1b}]}
Your command is incorrect. You need the full item name in quotes: