simple. turn on your monitor FIRST then turn on your computer. very quickly the video card BIOS runs and displays your video cards specs on the screen. this may take several reboots to write down all the information but it is the easiest way.
the otherway is to take the card out of the computer and look for the manufacturer and model numbers and then contact said manufacturer and ask them to look up the model numbers.
there are also some freeware, shareware, and payware software utilities that identify what components you have in your computer but they are not 100% reliable and are often hard to locate.