"It seems to me that you can never know if your success was due to your own hard work or some lucky breaks"
You may have a point. But, I'm really tried of people attributing all success to luck (and using this as an excuse as to why they aren't succeeding). It also discounts all of the hard work that someone does put into something successful.
If I sit here and do absolutely nothing, I won't ever have a successful startup. This is why I know it's not mostly due to luck. Luck implies that it happens to you with no intervention..like winning the lotto.