There is a layer property called isGroupLayer which returns a boolean value (true/false).
path = r"C:\temp\ds.mxd"
mxd = arcpy.mapping.MapDocument(path)
df = arcpy.mapping.ListDataFrames(mxd)
for frame in df:
for layer in arcpy.mapping.ListLayers(mxd, "*",frame):
print layer.name
print "isFeatureLayer - " + str(layer.isFeatureLayer)
print "isGroupLayer - " + str(layer.isGroupLayer)
del mxd