One of the most accessible and effective ways to identify assumptions we can turn into questions is to consider the whole spectrum of possible assumptions.
This includes: What we don't know, aren't clear about, aren't certain of, aren't convinced about, aren't confident in, what we're perplexed about, puzzled about, frustrated with, confused by, surprised about, frustrated by, what we don't have quick or easy explanations for, what seems unexpected, irrational, unreasonable.