def check_for_tag(instance, tag, desired_value):
	"""Check for a particular tag, and confirm that its value is what
	is equal to desired_value
	:type instance: boto.ec2.instance.Instance
	:param instance: The instance whose tags will be checked.
	
	:type tag: str
	:param tag: The name of the tag to be searched for
	
	:type desired_value: str
	:param desired_value: The string that indicates that kerberos is enabled.
	
	Returns: boolean
"""

	instance.update()
	if tag in instance.tags.keys():
		if instance.tags[tag] == desired_value:
			return True
	return False