Scope
Is the start condition clear? Is the finish condition clear? Are setup, review, cleanup, documentation and handoffs included? Have regulated or specialist steps been identified rather than estimated casually?
Resources and dependencies
Are the people, tools, space, materials and information available when needed? Which tasks are sequential? Which can truly run in parallel? Is there a shared bottleneck or approval point?
Calendar
Is the number expressed in hours, calendar days or working days? Are weekends, holidays, shifts, shutdowns, travel and external response windows represented correctly?
Uncertainty and learning
What could reasonably vary? Is the contingency visible? Is the estimate based on current historical data or on a guess? After completion, will actual time be recorded so the next estimate can improve?