How do you know if the motherboard is bad for sure?
Boot a live linux cd. If you get a desktop or video, your motherboard is fine. Look at the hard drive for defect.
now if i pull of hard drive?? can u tell it bad by lookin at it!!!
No, you can't. You might be able to tell if it was making sounds when plugged in, but you certainly can;t tell by pulling it out and looking at it.
Strip it down the bare essentials, and start swapping out components - starting with the power supply for a known working one. If you dont get any response, it's obviously broken. More than likely as Mike suggested your capacitors are bulging or leaking.
I think the only situation you can say it for sure are visual damages.
Examples would be heat damage visible via scorch marks, bulged-open or leaking capacitors, damage to the CPU socket (broken pins, bend pins beyond repairable), brown-black scorched transistors.
Everything else is never "for sure" - even if the BIOS is corrupted duo to an upgrade gone wrong you could still exchange the EEPROM (or whatever it is stored on on) with an equivalent one and try to boot up.
This approach worked for myself once by using an Abit EEPROM with an Chaintech mainboard ~ both were using Award BIOS, just different versions.
Bust it with a sledge hammer. You now know for sure ;)