type(o) == type(pow)

>>> import types
>>> types.BuiltinFunctionType
<type 'builtin_function_or_method'>

isintance(o, types.BuiltinFunctionType)

>>> import types
>>> isinstance(pow, types.BuiltinFunctionType)
True
>>> def a():
    pass
>>> isinstance(a, types.BuiltinFunctionType)
False

import __builtin__
o in __builtin__.__dict__.values()

o in __builtins__.__dict__.values()

>>> pow in __builtins__.__dict__.values()
True
>>> def a():
...   pass
...
>>> a in __builtins__.__dict__.values()
False
>>>