diff --git a/skills/code-review-and-quality/SKILL.md b/skills/code-review-and-quality/SKILL.md
index f594590a..08209ff2 100644
--- a/skills/code-review-and-quality/SKILL.md
+++ b/skills/code-review-and-quality/SKILL.md
@@ -161,6 +161,8 @@ Tests reveal intent and coverage:
 - Would the tests catch a regression if the code changed?
 ```
 
+Answer the last question by experiment, not by reading. Invert one condition the change adds (drop a negation, swap `&&` for `||`), run the suite, then restore the file from a copy. A mutation that stays green is a finding: name the test case that is missing. For a project-wide mutation score, see `constraint-driven-development`.
+
 ### Step 3: Review the Implementation
 
 Walk through the code with the five axes in mind: