I have heard that intelligence agencies and hackers can illicitly gain access to a computer's webcam and microphone and keep a close eye on people using suspicious programs or malware.
Is there any way to check if such thing is happening to you and are there any mechanisms to avoid it on platforms like Mac OS, Linux and Windows?
If, despite the implausibility of it, you are worried that the FBI has hacked your camera, then I'm afraid there won't be much you can do to gain back control of your camera. Therefore I can only suggest a low-tech solution: a camera cover. This is the sort of thing you are looking for:
https://www.amazon.com/C-Slide-Sliding-Computers-Chromebooks-Consoles/dp/B00AZ639VG
Aside from physically disabling the camera hardware, on some devices a webcam can be disabled in BIOS/UEFI, thus preventing access to the hardware from the operating system.
You may not be able to tell whether the camera is actually powered and actively recording just by looking at it. On some devices, there is a faint LED indicator which tells you the webcam is active, but this is unreliable.
If you believe you are being remotely monitored, you can use a packet sniffer, such as Wireshark, to check outbound traffic. Video streams are typically large and tend to stand out.
A camera cover or tape over the camera on a laptop or 2-in-1 desktop.
On a desktop (which typically requires a separately connected web-cam), just disconnect the webcam completely. If the webcam requires another source of power from the outlet, disconnect it from the outlet as well.
