Webcam test
See exactly what your camera sees before the meeting does.
Private by design: the picture goes from your camera to this page and nowhere else. Nothing is recorded, nothing is uploaded, and the camera is released when you stop.
Black screen? Check for a physical privacy shutter (lots of laptops have a tiny slider over the lens), then check another app isn't already using the camera. Grainy, purple, or refusing to focus? Bring it in — camera modules are replaceable.