There is no good software way to do this.
Monitoring outgoing traffic on computer with the camera is no good solution as your traffic stats may be faked. If someone gains such a good access to your webcam to disable the light, faking traffic stats is not a huge step away. You could measure the traffic on your router, but then you would need to read the stats on another computer, and, besides that it is extremely inconvenient, the router can be attacked, too.
I protect me from camfecting with a thick tape. I can pull it (partly) off when I need it, and as my cellphone already includes a microphone that can tap me, I don't care for the webcam's mic, at least most times.
If you also cared for your mic, or had a tin foil hat, I'd suggest to completely disable the microphone and camera on the hardware side, most times by removing it. The NSA recommends to remove the camera (pdf). In the pdf are additional tips on how to protect yourself from camfecting:
The best way to disable an integrated iSight camera is to have an
Apple-certified technician remove it.
Placing opaque tape over the
camera is less secure but still helpful. A less persistent but still
helpful method is to remove
/System/Library/Quicktime/QuicktimeUSBVDCDigitizer.component, which will prevent some
programs from accessing the camera.
To disable the microphone, even
if it means crippling the sound system, remove the following file from
Don't overestimate the software-side solutions above. Through a privilege escalation attack they can be defeated.
You can buy yourself a cheap usb webcam and use that instead of the built in one, and when you don't need it you can simply disconnect it.