Can Proctoring Software See My External Monitor on Mac?

Yes. Almost every major proctor (Proctorio, Honorlock, ProctorU, Respondus Monitor, ProctorTrack, Examus, OnVUE) checks for connected external displays and either blocks the exam or flags the session.

macOS exposes a list of attached displays through the IOKit framework and through Cocoa's NSScreen API. Proctors enumerate these. If more than one screen is attached, the proctor reaches a decision: block, warn, or flag. The exact behavior varies (Proctorio = warn, OnVUE = block, Respondus Monitor = often warn). The fix is simple: unplug the second monitor before the exam. The overlay sits on whatever is your active display and uses window privacy regardless of how many displays are connected, so the multi-monitor check is independent of the overlay.

In clamshell with an external monitor, you're using the external display. Proctors still detect it (one display attached, but it is the external).

Yes - Sidecar registers as an attached display and gets flagged.

Risky. The proctor may re-check periodically. The safe path is to unplug before the exam starts.