Who's On My Wifi
Who Is On My Wifi detects intruders
on your wireless network.
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Monitoring Wireless Networks through Log Checking of the Router or a Switch
The easiest and most common way to Monitor who is on a wireless network is simply to go into the Wireless Router itself.
Inside the Wireless Router, you can look at who is connected through a few different methods. Because most Wireless Routers also have MAC Filtering, which allows you to filter Wireless access to only specific MAC Addresses of computers, then normally on this screen you'll also see a button that says something like "Wireless Client MAC List". This usually gives you a list of all currently connected computer's MAC Addresses and sometimes computer names or most recent IP as well.
If your Wireless Router is also acting as a DHCP server, then sometimes you'll also see a list for "DHCP Clients Table". This will usually gives you a list of computers that have connected recently.
If you're in a larger organization, it's usually best to monitor this through the Firewall gateway(s). That's because most computers at some point will need to get out to the public internet, so all computers should be listed in the ARP Cache Table of most higher level firewalls.
If you're in a larger organization and there are computers that do not connect to the public internet, you could always log in to the managed switches and check their ARP Cache logs as well. This will usually give you the MAC Address of all computers that have connected recently.
Monitoring a Network through Wireless Router and Switch logs is probably the most common way that people think of when they talk about identifying computers on their network. It has severe limitations, but it's free and requires no 3rd party software to achieve.