If there is more than one DIMM in the system, you could try swapping them. Possibly by rearranging the memory map it will allow you boot as far as running diagnostics on the other DIMM. Other than that, the only other method is a dedicated diagnostic tool for the DIMM alone and as I stated earlier, they are VERY expensive, think in the $US 100K+ region!
Even if you could isolate one IC on the DIMM, you still need to find a matching replacement and you need SMD soldering equipment to change it.
DRAMS are complicated VLSI devices, there is no easy way to test them I'm afraid.
Brian.