How to distinguish from a bad flash and a dead mobo?

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How to distinguish from a bad flash and a dead mobo?

When a system has a bad flash, what are the symptoms?
It will beep when RAM is missing?

[cp]

neither of the board would beep when RAM is missing. The beeping is a function that is done in the bios. And to execute this kind of function the board has to be working and equipped with a working processor. otherwise: no beeping at all.
to distinguish between a bad flash and a dead mobo you have to take out the bios chip and reflash it with the correct bios again (hotflash, external programmer, ide programmer, isa programmer, etc.).

it is possible that an eeprom can erase by itself?
It happened twice to me. (i can't believe that the motherboard broken so suddendly)

First time, i tried to put a 60gb hdd on a k6 (limited to 32gb)
Once installed the dynamic drive override on the disc, at windows boot, it appeared a "bios virus warning" and system was no more able to turn on.

Second time, it stopped to work without a reason.

[cp]

it is possible that an eeprom can erase by itself?

no. an eeprom can not erase by itself. and even though it has inbuild write and erase routines they need special command sequences to prevent accidential destruction of data.
there always is a reason why hardware stops working. with mainboard the reason is most likely NOT the bios with one exception: something went wrong during flashing the bios (wrong file, corrupted file, ...)