If you want to know whether a material is used by any objects in the scene, you can check the UsedBy property.

Here’s a Python snippet that finds unused materials in the current material library:

from siutils import si

if Application.Version().split('.')[0]>= "11":
	si = si()					# win32com.client.Dispatch('XSI.Application')

from siutils import log		# LogMessage
from siutils import disp	# win32com.client.Dispatch
from siutils import C		# win32com.client.constants


matlib = si.ActiveProject.ActiveScene.ActiveMaterialLibrary
for mat in matlib.Items:
	if mat.UsedBy.Count == 0:
		log( '%s <Not used>' % mat.Name )