How can I testfor a named entity?

The name argument shoudn't be capitalized, and also unless you are testing for the mob in a specific area, r argument isn't required, nor the ~s

Try this:

/testfor @e[name=Zombie_Boss]

/testfor @e[type=Zombie,Name=Zombie_Boss]

You need to place a comparator and then a repeater leading into a command block with the command.

/testfor @e {CustomName:"Zombie Boss"}