This is a big open topic, but I'll talk about my top method.

I have a prior that our capitalist, semi-open market is thorough and that if an idea is economically feasible, someone else is doing it / working on it. So when I come up with a new good idea, I assume someone else has already thought of it and begin researching why it hasn't been done already. Once that research is done, I'll know not only if it is a good idea or a bad idea but why it is which, and a hint of what it would take to turn it from a bad idea into a good idea. Often these good ideas have been tried / considered before but we may have a local comparative advantage that makes it practical here were it was not elsewhere (legislation, better technology, cheaper labor, costlier labor... )

For example: inland, non-directional, shallow oil, drilling rigs use a very primitive method to survey their well bore. Daydreaming during my undergrad I came up with a alternative method that would provide results orders of magnitudes more accurate. I put together my hypothesis that this was not already in use because: this was a niche market and the components were too costly / poor quality before the smartphone boom. My hypothesis was wrong, a company had a fifteen year old patent on the method and it was being marketed (along with a highly synergistic product line) to offshore drilling rigs. It was a good idea, so good of an idea that it made someone a lot of money 15 years ago and made offshore drilling a lot safer, but it wasn't a good idea for me.