On very long cross country trips, I listen to ATC "call out" traffic to other aircraft and then I listen to see if the other pilots actually sees the "called out" aircraft and respond to ATC that they have the other aircraft in sight. I make a running tally of aircraft called out by ATC and aircraft actually seen by the other pilots and responded back to ATC that the other aircraft has been seen. The numbers do not look good. I know that there are some pilots that see the other aircraft, but do not respond, but I am assuming most pilots that are advised of other aircraft by ATC will respond if/when they see the other aircraft. It looks like around 20% of called out aircraft are actually spotted during a en route flight and are identified verbally back to ATC. However, the number of aircraft "identified" is much higher when the other airplanes are in the pattern. I do have to say that my own personal percentage of spotted flying aircraft is pretty low of the "called out" aircraft by ATC, but my flight instructor has an uncanny ability to spot flying aircraft and he would have been A WWI ace.