I don't believe that you tell that on the Apple site. It will show if the computer is still under warranty.
If you are buying on CL make sure you see the computer in person and take a few minutes to power it up and watch it boot and check it out.
Last year I found what was advertised as a G5 PowerMac 2.0 and worked out a deal to buy it. When I went to see the computer I noticed that the fans ran excessively loud and when I looked closely at the specs under About this Mac I realized that it showed a single 2.0 GHz processor. Looking inside the motherboard was only for a single processor. So somehow it was a 1.8 Single processor motherboard either over clocked or with a 2.0 processor. I bailed on that deal in a hurry.