How to know if the damage is OMSI side or BCS side?

From GO Group Virtual Wiki

How to know if the bus is damage or if the damage is the BBS type (what we care or another type)

They don't turn on/off at all. If they blink fastly, then it's the OMSI internal damage system which can be fixed using the OMSI menu.
Of only one side is broken then full beams don't work, if both sides are broken then headlight switch doesn't work at all. If only one side is broken, you can usually find which side with indicators as headlight and indicator 90% times break at the same time.
They open and closes very slowly.
Heater controls do not affect anything
Engine doesn't turn on or off
Electronics doesn't turn on or off

If it's not one of these damages, then it's OMSI damage, which can be fixed in-game using the wrench tool, can't be done in BCS.