I don't think this is always the case. I think that many good ideas are obviously good regardless of their origin and even in the absence of external evidence. You can tell those ideas are good because they solve real problems in people's lives (Ebay, Yahoo, Craigslist, Amazon, Hotmail, etc, are all based on obviously good ideas).
To use Paul's analogy, some pots of gold are at the top of mountains, but others are visible on the flatlands, and only speed of execution determines who will win them.
Indeed it's often impossible to know if a new idea could lead to a profitable business (or how profitable that business would be), but at least you can have a good hunch that the idea would be something people want.
The original Yahoo idea was a centralized directory of web pages, wasn't it?
To use Paul's analogy, some pots of gold are at the top of mountains, but others are visible on the flatlands, and only speed of execution determines who will win them.