Look at the back of your computer. If your speakers are plugged into one of the lower expansion slots, you have a sound card. If it plugs in near the USB ports, network, etc. then you have onboard sound.
Go to Device Manager and expand the 'sound' section and you can find out what you have that way. Or... if you know what motherboard you have, look it up on google to see what sound chip it has onboard. If you have a dedicated sound card, just open the case and look at it.