How and when did you find out that your idea/ startup is worth pursuing?

You wake up, get amazing idea and feel like it's the next big thing. You get cracking on it immediately and then also start building a team. Everything is going well. Soon, you realise that you need to commit your full time to this and cannot juggle between a job and this venture. How do you decide what to do from here on? What I'm asking is, which metrics do you use to validate your idea? How do you know if your product is actually meritorious?