I've reviewed network tools with some suggestions but Rogue Scanner may be a better way to find out whether your network has any intruders on it. Before running this program you can assemble a list of every PC and device on your network. Thereafter by deploying RogueScanner you will have a list of every device on your network, including routers, printers, PCs and items. The IP and MAC addresses are listed but even more it investiages even further to find additional information such as whether the device is a workstation, printer, server, router or PC, as well as the manufacturer and model number.
You can then compare notes between your list and what the program finds with the list of devices that you know are safe and secure. If there is something that does not look famaliar then your system has been compromised.