We read public reviews and forum posts where people describe what a product fails to do, extract each complaint, and group the complaints that describe the same problem. A problem that several products share, from several unrelated users, is a market gap rather than one vendor's bug.
Each idea on this site is written against one of those problems and carries its evidence: how many complaints, how many products, and the complaints themselves. The Traction feed adds the other half of validation: products founders already report revenue for, screened against their own comment threads.