Pull out the good ol' user guide and read... or google up the model of your adaptor. If the computers are working properly and an OS is installed, check for it in your OS. Or, you could transfer a big file and see how fast it goes lol. Either way, I'd get the gigabit switch, more future-proof incase you want to buy a new computer with gigabit networking. Besides, you can buy a gigabit adaptor for $20 and make your existing PC's gigabit.