SoftPerfect Wifi Guard is a simple tool which makes it easy to detect intruders on your local network.
The program requires no installation (the Windows build, anyway) - just run it, and SoftPerfect Wifi Guard will quickly scan your network and display any devices it finds. Anything that's clearly a part of the infrastructure, like the PC you're working on or your router will be flagged as green, but everything else will be marked as red: a potential intruder.
If you recognise a device as legitimate - a network storage device, say, or another system on your network - then right-click it, select Properties, check "I know this computer or device" and it'll be removed from the danger list. Which, initially at list, will probably dwindle to nothing at all.
SoftPerfect Wifi Guard will scan your system every half hour, though, and if anything new and unknown has connected in the meantime then you'll be informed via a suitable pop-up alert.
Plainly this is no substitute for proper wireless network security, such as using the highest levels of encryption supported by all your network devices.
SoftPerfect Wifi Guard does provide a simple extra layer of protection, though. It's also lightweight and entirely free (no adware concerns), so if you're interested then you can try the program without suffering any hassles, penalties or problems.