How will I know when people are ready to pay?

The easiest thing to do of course, is just throw out a paywall of some kind and just see who pays. But that takes engineering resources which you don’t want to waste by building upgrade paths unless you’re certain that folks with their credit cards are standing by. You could create an “Upgrade” link in the software and track how many people click on it, and when they do it just pops up with a “coming soon” message…

— The Minimum Viable Product Lifecycle - knowing when to jump from a minimum viable product to a business.