Ahem... *Raises hand* I.. through some ways.. have turned on people's webcams just to see what's going on. It isn't that hard at all to do.
But since the ways of doing it are slightly illegal I'm not willing to say how or what to even Google.
Just know that it can be done without any detection of any software. And I'm not talking about remote connection or teamviewer either. The best 'solution' is to keep a strip of tape over your webcam and never remove it until you want to use your cam.
There is no zero day exploit out there that does it. Almost all access to machines remotely to activate any hardware what-so-ever is done through some sort of malicious software. Typically installed by the end user by some social engineering or them visiting malicious sites that elevate code from basic programming flaws like stack over flows and such developers still can't seem to fix (well bad developers), or from something embedded in pirated software.
You also can detect what is running on your system, the problem is you have to know what to look for. Everything in Windows, OS X, or Linux has a process (service) name and is taking up resources.
So, as long as you execute "best practices," on your computer and have decent up-to-date software for security you most likely will not succumb to such an attack of having your webcam controlled. If you are that paranoid perhaps you should stop using Windows, or just put a band-aid over the web cam itself like I previously mentioned.
