Some concepts can be validated by talking to potential customers/consumers/influencers, but for some concepts entrepreneurs have to take a leap of faith.
For concepts where asking customers/consumers/influencers is unlikely to provide definitive answers, the entrepreneur should rely on getting a deep understanding of the domain, the dynamics of the industry, the pain points, the need & opportunity gaps, the process of decision making, etc. Once you have a deep understanding you build a hypothesis and make some assumptions on how the market would respond to the concept. e.g. what percentage of consumes who you reach would consider buying, how much would they pay, why would they buy, how many will repeat purchase, etc. Once you have your base assumptions ready, discuss it with a number and people to get a sense if they feel that your assumptions are practical. [Note: Just because many people feel that your assumptions are practical is no guarantee of success. However, if many feel that the assumptions are impractical, it could be a warning signal and you may want to revisit your assumptions.].
Once you are comfortable with a set of assumptions, and after those are endorsed by a few people, it is prudent to do multiple versions of the scenario. Some optimistic and some pessimistic scenarios. While the optimistic are largely to give you a feel-good factor, the pessimistic ones are the ones that will give you a feel of how wrong you need to be for your venture to fail.
For concepts that can be tested, talking to enough customers/consumers/influencers to get multiple perspectives is essential. Diversity of audience segments is important to give you a sense of the receptiveness of the concept within a broader audience profile. There are enough qualitative and quantities techniques that you could consider. For most startups, who are keen on getting a first-cut feel of customer/consumer responses, doing basic research through students is a good way to begin.
Any case, there is no better way than for the entrepreneur to have one-on-one conversations with potential customers/consumers.