how do I know if my webcam on my Macbook Pro is hacked?

Well, he is talking of a hack. So, a hacker probably would try to avoid that the green pilot light is activated. What has been done before. I think that is his point...

A link of how it has been done, as part of a security test in John Hopkins University, can be found in:

https://www.intego.com/mac-security-blog/your-macs-camera-can-be-hacked/

As I see it, you can't be totally sure - you can trust your diverse security measures, and take all, but there seems no nonhackable way to achieve it. Except one: cover it.

If the camera is enabled, you will see a green pilot light glowing on its right. Run the Photo Booth app to see how it looks like.

And the microphone is located below the left speaker grille, NOT next to the camera. If you have a MBP with anti-glare screen, the little grille next to the camera in the aluminum frame of the lid is the ambient light sensor, NOT the microphone.

Not normally. However, certain Adobe Flash pages can be configured to use the video camera, and if you are visiting an Adobe Flash page that is, and you had setup the camera before on another page, it is possible the software is making a call on the camera again. Photobooth, Quicktime, iChat, Skype all speak to the camera in varying degrees. Additionally there is a built-in microphone next to the camera which may be called by any audio-input software. If you want to be secure, just put a sticky on top of the camera. I would though upgrade to 10.7.3 if you are worried about being hacked. There has been a malware that affects 10.7 through 10.7.2, and 10.6 through 10.6.7, and 10.5.7 and earlier: https://discussions.apple.com/docs/DOC-3261

That depends on what you mean by "hacked." When the light is on, the camera is active. If you don't know why that's happening, you have to figure it out. Most likely, it's third-party software that you installed and that isn't behaving as you expect.

Very unlikely, and only possible if you installed some software that would permit it.