The unitTag for bosses is 'bossX' where X is a number between 1 and MAX_BOSSES (currently 6).
Code:
DoesUnitExist('boss1')
will return true if you are in range of a boss with the unitTag 'boss1'.
Code:
GetUnitName('boss1')
will return the name of the unit with the unitTag 'boss1' if it exists.