I am not sure that it is good idea to accept a type and an instance, but if it is a goal, is_dataclass() should be defined as:

def is_dataclass(obj):
    cls = obj if isinstance(obj, type) else type(obj)
    return hasattr(cls, _FIELDS)

_is_dataclass_instance() should be changed too:

def _is_dataclass_instance(obj):
    return hasattr(type(obj), _FIELDS)