In my former home in FL, we used a motion sensor and a camera mounted to watch the front walk. The camera also had motion sensing (or rather the digitizing software did), but it was easier to use the separate sensor. We also had one inside the front door, in the hallway. If the motion sensor went off first, and then the door was opened (door sensor), we were coming in, and the opposite for going out.
We also used internal "occupancy" status as well as the alarm de/activation for confirmation to help determine whether we were coming or going and various states of the house and systems. It does take some logic, but it's do-able.
You can also use the camera for security or record visitors, etc. We had the motion sensor also trigger some archiving of digital camera footage, both video and stills taken in a timed sequence. We did catch the UPS delivery guy picking his nose once! ;-)