Checking if your instancefile files actually exist...Cos sometimes...they don't! Stolen from MikeB
Paste this into a Python node:
import os
node = hou.pwd()
geo = node.geometry()
geo.addAttrib(hou.attribType.Point,'exists',0)
for pt in geo.points():
file = pt.attribValue('instancefile')
if os.path.isfile(file):
pt.setAttribValue('exists',1)
else:
pt.setAttribValue('exists',0)