Well depending on the type of BIOS, you will press a certain key during the initial boot prosses, the key could be:
1) Del
2) F2
3) Esc
4) F10
Usually it will be one of the above. Its best to keep tapping the key as the system boots so it doesn't blow by the chance to access the bios. And usually if you watch the screen during the boot process it tells you the key to press to enter the bios.
Look for a section called "Integrated Peripherals" or similar. Inside should be settings called "Onboard Serial Port". If they have numbers after them similar to "3F8/IRQ4", they are enabled. Otherwise it will say "Disabled".