How do you choose a business idea?

What are the metrics that you use to decide a particular idea is worth pursuing?
From where do you collect data?
Do you directly reach out to potential customers, look at sales volumes on e-commerce sites, frequency of discussions about a topic in communities?

I'm rethinking the content format of explodingniches.com. I want to make it concise and present only the metrics that bootstappers use to make decision.