We have psychometric tests for this stuff. So yes, we can tell whether people believe they are fulfilled, and for this purpose that's the same as knowing whether they are in fact fulfilled.
And yes, luck is a massive factor. Warren's strategy sounds good, but it's the same strategy lots of people have, without getting his success. We might as well listen to the anomalous 110 year old who tells us he puts it down to a long walk every afternoon. No doubt walking doesn't hurt, but it's not why he's 110, that's just blind luck.
Survivorship bias is a huge problem. If 500 people all pick one of twenty strategies at random, and all but one of the 500 fails, we would be wrong to assume that therefore the strategy chosen by that one person works and the other nineteen do not. But that's exactly what survivorship bias causes us to assume.