Sorry for being so obvious, but you might try tracing from each actor position to your POV (camera position), then checking if the trace was unsuccessful (nothing occludes the visibility) and it lays within the frustum cone. This is an ugly hack, I might guess, but I am not familiar with Unreal’s graphical pipeline.
If you stick to Tracing, Rama has this covered in a tutorial: https://wiki.unrealengine.com/Trace_Functions