No, you can’t stop engineers from testing aero in their heads, but I would imagine teams are required to keep logs and documentation of development work/wind tunnel time/computer simulations for FIA to inspect at any time. It’s nothing new either, each team has a wind tunnel time and CFD allocation per season, and there’s no development allowed over the summer break. I suppose in the end you have to trust that FIA is enforcing these rules properly, but you can never know for sure.