If you just see the open ends of the ports surrounded by labels that tell you which port is which, your plate is pre-installed.
Note also that if you have a mid-range to high-end board made in the last few years, the IO plate may come pre-attached to the motherboard, saving this step. Look at the area where the USB and audio ports are on your motherboard. If the ports look bare, with no cover labeling them and hiding port housings, then you'll have to install the plate yourself. If you just see the open ends of the ports surrounded by labels that tell you which port is which, your plate is pre-installed.