They analyze fecal excrements with a stick. After few mixes the more it looks like the Google logo the better the employer is. Image matching is performed via a cluster of slaves voting from 0 to 9.
Well... apart from joke, it is a very bad sign when you start to "rate" humans, I think that all this meters at Google are the reason why most of us thing that Apple accomplished good design and beautiful creations. Now thing about Google: Yes, smart engineering but there is some ingredient missing. Sometimes good things come exactly from the guys Google will reject at the first stage of the interview.