Windows 10's May 2019 feature update delivered a significant improvement to the webcam settings page. Windows now gives you more privacy by letting you see which apps are using your camera. If your webcam indicator lights up unexpectedly, you should be able to quickly identify the culprit.
The list of apps resides within Windows' existing Camera Privacy settings page. To find it, open the Settings app from the Start menu (or use the Win+I keyboard shortcut). From the Settings homepage, click the "Privacy" tile and then the "Camera" page in the left navigation menu.
The page is divided into two sections: Microsoft Store apps and desktop apps. More control is available for Microsoft Store apps, as these can only access your camera through well-defined interfaces that are guarded by Windows permissions. You can prevent individual Store apps from accessing your camera by using the toggle buttons on the page.
Below each app's name, you'll see the time that it last used your camera. If an app is listed but no time is displayed, it can access your camera but has not yet done so. Apps which are currently using your camera will display "Currently using" in bright yellow text.
The desktop apps section works in a similar way, except you can't prevent desktop apps (traditional Windows programs) from using your camera. The list displays all programs which are known to have used your camera in the past. It will also display "Currently using" to highlight apps which are actively streaming video.
Unfortunately, you can't necessarily rely on the contents of this screen. By virtue of their nature, desktop apps are able to access your camera hardware directly, bypassing Windows' regular APIs. This means that a malicious app could stream your camera feed without notifying Windows, so it won't show up in the list. Generally, most respectable programs will appear here, but Microsoft's own documentation warns of the possibility that some won't.