A checklist with missing items is worse than no checklist at all. It gives false confidence.
Something might pass every check on your list but still fail catastrophically on a dimension you forgot to include.
To validate completeness, imagine hypothetical cases where something checks all your boxes but is clearly deficient. What's missing?
For example, a software deployment checklist might include "Tests pass" and "Code reviewed" but miss "Database backup created." That omission could be disastrous.
Completeness doesn't mean listing every trivial detail. Focus on significant dimensions of merit that are easy to overlook.
Have others review your checklist. Fresh eyes often spot the obvious gaps you've become blind to.
References
Scriven, M. (2000). The logic and methodology of checklists.