If there are many California tortoiseshell caterpillars on snowbrush, it is usually not too difficult to locate them as they are mostly black with spines against green leaves and/or white blossoms in the background.  Always remember that females lay eggs in large clutches.  Scan the bush for larvae; but don't spend too much time looking.  If you find one larva, you may very well find many; but if don't find any (which is more often the case) move on to another bush.