The end result of any effort will be the adoption of a feature or product by customers. This is the true North Star of any product leader. This is called Demand Validation.
There is no clear method for 100 percent validated idea. There is not a situation whereby an idea has been successfully validated. There is no True or False.
The effort to validate is to reduce risk vs. the feasibility to build the feature. Once the teams are confident in the risk, you can start the development process. Not everyone will be comfortable with this approach, and collaborating early in the process will help people understand what was left out from the feature to achieve a confident validation level.
Nothing is 100 percent validated. Validation depends on the risk aversion within your organization. Risk aversion is correlated to how confident the teams are with building and developing. As engineering teams become more competent with XP, Scrum, Kanban et al., the organization becomes less risk-averse.