Write down and clarify your current analysis of the problem or situation you're dealing with. Don't focus on assumptions yet, but the summary of your thoughts.
Set a timer for 10 minutes, and write down all the assumptions you've made in your analysis. Ask the standard journalistic questions: Who influences the situation? What do they want? How are they going to act? When? Where? Why do we think so?
Watch out for three key phrases: will always, will never, would have to be. They are signs of unchallenged ideas/assumptions. Fix that. Also: "based on" and "generally the case" need to be challenged too!
After listing all assumptions, examine them: Am I confident that this assumption is correct? Why? How confident am I? When might this assumption be untrue? Could it have been true in the past but not any longer? If it's invalid, what happens to the analysis?
Place each assumption in one of three categories:
Basically solid (S)
Correct with caveats (C)
Unsupported or questionable – "key uncertainties" (U)
Refine the list. Remove bad assumptions. Then focus hard on assumptions that change the conclusion if they are wrong.
Turn key uncertainties into research projects to gather more information on, if they have material impact on the conclusion
Review the results of this key assumption check: How good is your evidence? How reliable are your sources for this evidence? How likely is it THAT YOUR CONCLUSION IS WRONG? Focus on how likely it is you might be wrong – this offsets positivity bias.