Simulation is widely used in autonomous-driving validation, but matching task outputs between real and synthetic images does not guarantee that a system relies on the same visual evidence. This creates a validation gap for transfer, especially in safety-critical perception functions. We present Concept-Decisive Feature Fidelity (C-DFF), a concept-level mechanism-parity diagnostic for paired real-synthetic evaluation. Starting from decisive evidence extracted from a frozen system-under-test (SUT), C-DFF projects that evidence into a shared semantic concept space using off-the-shelf segmentation and compares real and synthetic concept-importance vectors. The result is not only a scalar mechanism-parity score, but also a concept-shift diagnostic that identifies which scene concepts become more or less decisive in simulation. We evaluate C-DFF on KITTI $\leftrightarrow$ Virtual KITTI 2 with 2126 matched pairs across three frozen SUTs (steering regression, YOLOP drivable-area, YOLOP lane-line). Across these tasks, C-DFF is SUT-discriminative and yields interpretable concept-level summaries that support validation triage and simulator debugging. When reported alongside task-specific output-value context metrics, C-DFF shows that output agreement does not by itself establish mechanism parity. These results position C-DFF as a practical semantic diagnostic layer for validation and verification workflows in simulation-based AV testing.

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