If asked to identify the form factor of a motherboard, the two criteria you can use to help identify the board are the location of the CPU and expansion slots and the type of power connection used...
P1 motherboards are "Socket 7", it should be written on the CPU socket. Also, they only have PCI and ISA expansion slots, AGP and PCI-E Video cards slots never appear on a P1 motherboard. The ram...
BIOS is not dependent on the form factor (ATX) of the motherboard. BIOS are chips with information on how to run certain hardware, these are usually American Megatrends, Award, or Phoenix.