Install iftop.

sudo apt-get install iftop

This will install a little package which will let you listen for incoming connections. Of course, this could have its limitations if you are using this to detect unwanted intruders in your network who may be able to fly under the radar. But for the use case where you think a general user is using your wifi or something, this will do the trick. Use iftop as follows (Must be run as root) -

sudo iftop