What you are looking for are the LVARs ‘BTN_LS_1_FILTER_ACTIVE’ (captain) and ‘BTN_LS_2_FILTER_ACTIVE’ (first officer).
Access to them is done via WASM. In my case, I use SimConnect to communicate with my WASM module. A good sample is provided here. It’s written in C++ but it’s easy to convert to C# (as I have done).
Regards,
SpiralCharger