A two minute check before a call
Most video call problems are discovered in the first thirty seconds of the call itself, in front of everyone. Running both devices here first catches almost all of them.
Framing
Your eyes should sit roughly a third of the way down the frame, with a small gap above your head. If you are looking down at a laptop screen, raise the machine on a stack of books until the camera is at eye level. This single change does more for how you come across on a call than any hardware upgrade.
Lighting
Put your main light source in front of you, not behind. If the frame rate reading is low, that is the room, not the camera. A window behind you turns you into a silhouette, because the camera exposes for the brightest part of the scene. If the frame rate reading is low, that is usually the room being too dark rather than a slow camera: sensors hold each frame open longer in low light, which drops the rate.
Background
Check what is actually behind you in the preview. The unmirror button is useful here, because it shows the scene the way other people will see it, with any text the right way round.
Audio
Speak at the volume you will use on the call and watch the meter. It should move well into the middle without staying in the red. If you are using laptop speakers rather than headphones, the browser's echo cancellation is doing real work; wearing headphones removes the problem entirely and is the easiest way to sound more professional.
Test the setup you will actually use. If you plan to plug in a headset for the call, plug it in before running this test. Connecting a headset changes the default input device, and a test done without it proves nothing about the call you are about to join.
If one works and the other does not
That split is diagnostic. Camera and microphone permissions are granted separately by browsers and separately again by Windows and macOS. A working camera with a dead mic points at the microphone side. If your camera works here but your microphone does not, look at the microphone permission specifically rather than assuming a general fault.
Other checks
Common questions
Why test both at once?
Because that is how a video call uses them. Some laptops route the camera and microphone through the same USB controller, so a fault can appear only when both are active together.
Should I test with my headset plugged in?
Test the exact setup you plan to use on the call, including the headset. Plugging in a headset changes which microphone is selected, and that switch is a common cause of a call where nobody can hear you.
Nothing is recorded, right?
Correct. Both streams are handled by the browser and rendered live. There is no recording and no upload.