ECC memory has two advantages:
- It is registered, meaning that there is a register before other components on the chip. This is supposed to remove electrical load from the memory controller. This is true of all RDIMMs, not just ECC RAM.
- It can detect errors, and if not recover from them at least report that they happened
Given this, it is actually very difficult to determine whether you would have benefited from ECC ram without having ECC ram. By definition you cannot log the failure to detect an error, and you certainly don't have data on whether the error which may or may not have happened was the result of the memory controller messing up.
That said, if you run memtest, you will determine a couple things. If you find no errors, either you need ECC RAM, or the problem is with something else (so if you rule absolutely every piece of hardware and software out as the cause, you have shown the need for ECC RAM). If you find consistent errors, chances are the RAM is bad and just needs to be replaced. If you find inconsistent errors, the CPU might be bad, or you might need ECC RAM. If you find that memtest86 crashes, either the lowest-order DIMM is bad, or the CPU is bad, or you need ECC RAM.
Regardless, this is very tricky to definitely show. ECC RAM is most useful in applications where invisible errors in calculations are likely to cause extreme problems, or in applications where the sheer quantity of RAM combined with other conditions makes errors statistically likely. However, these criteria themselves are fuzzy and subjective, so it follows that there isn't really an objective criterion for this.