I hadn't heard of this either, but a quick google turned up a description of false parity RAM: http://en.wikipedia.org/wiki/R...
TLazy;DR: To save cost where parity RAM was required by the hardware but not by the operator, modules existed that would calculate the parity bit upon reading the RAM, rather than storing the parity bit. I don't see any evidence that this type of module ever existed for ECC though.
To make sure memory is ECC, it's probably sufficient to count the memory chips on a DIMM. If there are 9 or 18 (or even 36, if it's a particularly large DIMM) identically-marked chips, that's ECC. If there are 4, 8, 16, or 32 chips, then it's probably not. If one of the chips is marked differently than the others, it might be a little more complicated; it might be possible that it's a different memory chip (e.g. if there are 4 x16 memory chips, you'd only need one x8 to get a x72 ECC DIMM, so that last chip would be different). But it's also possible that it's buffered/registered memory, and the different chip is the buffer/register.
And an aside on the topic of buying RAM for yourself:
In general, I'm not a fan of cheaping out on memory. I did computer repair for a while, and it shocked me how many problems were caused by bad RAM - from the obvious ("my computer crashes every time I boot it") to less obvious ("every few days, an application crashes") to the rather insidious ("it was running fine, and now I can't mount my hard drive any more"). It got to the point where, when a computer came in with nonspecific symptoms like that, I'd open up the computer and peek at the RAM chips first. If they had no recognizable manufacturer, they were certainly garbage. If they were recognizable but not top-tier, they probably needed some stress testing on our RAM tester. And if they were the good stuff (Samsung always had my vote there, though it's hard to find because they don't sell directly to consumers), then it was probably something else.
That's also where I learned that things like memtest86 or other software diagnostic tools were basically useless too. Only the absolute worst memory would fail a test, even a looped test run for days. Most bad RAM was marginal - after all, it probably passed some manufacturing tests. We had a rather expensive (~$4k-8k) box that would test memory, doing things like varying the supply voltage or self-heating the RAM. When RAM is installed in your PC, you're still limited by the hardware - i.e. the voltage regulator and the memory controller - which probably keep the memory as close to nominal conditions as possible. Obviously, those machines are rather hard to come by, so you have to make do with software tests instead - but a pass on those just means I can't prove it's bad; it doesn't mean the memory is good. Even if I pass all memory testing, I'll still swap/remove/replace DIMMs in an attempt to find which one is bad, because it's often not obvious.