SDRAM problem

From time to time I must repair an equipment that has its own BIOS, CPU and SDRAM
This SDRAM has 256MB together and consists of 4 SDRAM modules(IC).
Sometimes equipment does not work.Then there are several possibilities for that.
Faulty CPU or BIOS or SDRAM.
Is there a method how I can find out that SDRAM is faulty? If so which module of those 4 SDRAM modules might be faulty?
Each SDRAM module has 66-pin Plastic TSOP(II) and is soldered on PCB.
or what method shall I use to check that the processor sends signal( e.g. address signal) to SDRAM?
Thank you for help

you would have to determine the address range that each SDRAM covers

then, if you have some kind of resident monitor or OS, you try writing and reading from addresses within each chip's range...whichever one comes back with an error or doesn't write correctly would be the faulty chip

Dose it have an OS?

It has Linux, but it is embeded Linux.It has BIOS and if SDRAM is faulty it does NOT start at ALL

Look up the BIOS and see if it has a RAM test.

How can I have a look at the BIOS?
It is Linux embeded device. No choice for such RAM test