The Raspberry Pi is a surprisingly useful tool to test the strength of your network. To add another tool to your network testing kit, Warberry Pi is a self-contained set of scripts that run automatically when you plug your Raspberry Pi into a ethernet port. The Warberry Pi has a ton of scripts installed that run when you connect it to a network. Just plug your Pi in, power it up and it will monitor network activity, collecting details like IP addresses and MAC addresses. It then logs all this info so you can gather it later over SSH or by checking the Pi itself. This is only meant to test your own network, but if you're curious about just how insecure your Wi-Fi network is, this is an interesting way to test it out. The Warberry Pi GitHub page has everything you'll need.
Warberry Pi [GitHub]