To start with, talk to people who're the target audience for this idea or the underlying problem behind it. Try to gauge if they face the same/similar problem(s) that your idea is going to address.

Ideally, having some sort of early financial commitment (e.g. crowd-funding, pre-orders etc.) from interested people, is a good indication that your idea solves a problem real & big enough that people will pay for it, which is the most essential thing for startup success.