Well...I can only tell you how I approach it when I suspect memory issues
I run Memtest86+, although I've heard that the Windows Memory Diagnostic is also trustworthy.
I put all modules I have in initially...as soon as I get an error (any error) during the Memtest run, I stop and start systematically experimenting.
If modules produce an error, I cut that in half and try two (assuming I have 4 motherboard RAM slots). If that produces errors, I go to simply 1 module, placed in a specific slot.
If I get no error with that module in that slot, I try another module in that slot.
I continue with this until I've assured myself that the slot is not the problem and that the individual modules all work fine in the known good slot.
I continue in this fashion...until I can conclude that all modules are good and each slot is good.
That leaves the motherboard RAM timings and the clock speed as potential culprits, along with module compatibility.
From the previous testing, I've noted the individual module timings...and the listed clock rate of the RAM. If the modules reflect different clock rates, that could be a problem re compatibility. If the timings are different, that may be impacted by whether the motherboard is successfully adjusting for such.
It's a very systematic process...takes a little patience...but eventually, I am led to correct conclusion.
I had some RAM that was listed as PC3200 that was running at a slower speed...mixed with PC3200 running at the stated speed. The fact that I started receiving various STOP errors indicateed a memory problem. Once I realized that two of the modules were not running at stated/expected speed, I chose to reduce the BIOS multiplier (downclock) since I preferred to use the maximum number of modules and was not at all concerned with whether the bus clock ran at 166 or 200.
Investigative work...that's what checking out suspected RAM problems is