Seeing the difference is not the problem. Whether or not the internals went through the ringer and, while no problems are apparent in testing, have extremely reduced lifespan is.
Based on what evidence? My job was technology implementation for 15 years in a majority Mac shop. We had on average 30-35 macs plus iPhones and iPads in use in later years. Our standard attrition was three years for most users. We opted for refurb purchases whenever possible. Never did I detect any pattern that would have suggested that the refurbished machines performed worse or with a reduced lifespan when compared with those machines purchased new.
