Well if your system was bought within the last two years you probably have a PCI-ex motherboard. If it was bought within the last year or so then I'm pretty sure your motherboard is PCI-ex. Since the industry has moved to the PCI-ex standard ATI and Nvidia have followed suit and now make very few AGP video cards anymore. However in order to avoid alienating tons of people who still own AGP motherboards both companies still produce a few token AGP cards. So basically if you're intent on not upgrading to a PCI-ex motherboard your video card options are limited. Coincidentally ATI just announced an AGP version of the X1950 Pro card I mentioned earlier.
Download this fantastic little application and unzip it to its own folder.
http://www.cpuid.com/download/cpu-z-137.zip
Double click on the 'cpuz.exe' file in the folder you extracted it to and voila.
Once the application is running click on 'Mainboard' tab and look for anything that might say 'AGP'. Usually this info is listed in the bottom section where it says 'Graphic Interface'.