How to Check if a Layer is PRESENT or ABSENT in a Project via PyQGIS?

len(QgsMapLayerRegistry.instance().mapLayersByName('the_Map_Layer_in_question')) != 0

Trueif layer exists, otherwise False

 boolean = False
 layers = QgsMapLayerRegistry.instance().mapLayers()
 for name, layer in layers.iteritems():
     if layer.name() == ('the_Map_Layer_in_question'):
         boolean = True    
 # if the map layer exists boolean == True, if it doesn't boolean == False