How do I know if a thermal jersey has enough wind protection?

Hold the front panel up to a light source or blow through it. If you can see light through the fabric or feel air passing through, it has minimal wind protection. Quality wind-blocking panels will be nearly opaque and block airflow completely. Also check the panel coverage — the best thermal jerseys have wind-blocking fabric on the full front, shoulders, and upper arms. Some budget models only have wind-blocking on the chest, leaving your shoulders and arms exposed to chill.