First Principles Thinking is a means to challenge assumptions and solve complex problems by breaking them down into their most basic elements and reassembling them from the ground up.
Identify and challenge assumptions with 'what if?' questions. Ask open 'what if?' questions to explore unexpected and even unrealistic possibilities that might help you to reframe the challenge.
Explain the problem to a child. Inspired by the Feynman Technique, see if you can simplify the challenge to explain it to a young child, and focus on communicating the 'core' of the issue.
Identify what you really know to be true. Try to separate out what you believe versus the basic facts at play.
Work with diverse teams. Another way to challenge assumptions is to work with people from different perspectives, this might include novices or new people to the challenge and/or people from cross-functional domains.