Watch for someone who leads with channel tactics before understanding the business question, who can't point to how they'd measure success beyond vanity metrics, or who wants to own execution across every channel personally instead of directing specialists. Vague answers about availability or an unwillingness to name a clear engagement scope are also warning signs.
A good fractional CMO should be comfortable being measured, comfortable saying no to tactics that don't fit the strategy, and clear about what happens if the engagement isn't working.