SDRAM problem

[Jane]

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 some signals( e.g. address signal) to SDRAM?
Thank you for help