How you know that mother board is bad

Exact model number of unit.

What is the unit doing?

how you know that mother board is bad

Well, there are only two ways that I can think of to determine whether a mother board is bad or not. The first one is a visual inspection. Do you see any burnt or melted areas or any other visual damage to the board?
The only other way to determine board as your problem, would be to have the tools and knowledge to test all other components of the unit, eliminating every one before replacing the board. A control board in most newer furnaces is the heart of the system, and is not easy to trouble shoot. So you basically need to know what areas of furnace to start your trouble shooting, and work your way back to the board. Good luck, and I suggest calling in a pro to check the system, before you start replacing parts that are not in need of replacement. Im my experience, the control board is less than five percent of furnace problems.