An Ethernet card has a jack similar in shape to that of a telephone jack although slightly larger in size.  Just look behind your machine and voila, you either have one or you don't.  If you don't you can easily get pci expansion card with ethernet for very little these days.

oh yeah another quick way to find out if you have an ethernet card is to go into Device manager in windows, look in network contollers and see if you have either a 10/100 ether net controller or a gigabit controller either one means you have ethernet.