If your kernel uses modules (which is highly likely), one way to determine whether a program is accessing your webcam is to look at the usage count of the module:
$ lsmod | grep uvcvideo
uvcvideo 90112 0
The 0 in the third field shows that nothing has any device open for a `uvcvideo`-controlled webcam (when `lsmod` ran). Of course you need to know exactly which module is responsible for your webcam; it's easy to check though, you'll see the output change while running a program such as Cheese.
Note that, strictly speaking, a positive count only means that something has opened a device, it doesn't mean images are being captured.