How to See Who's On Your Wi-Fi

Is your internet sluggish? If you suspect a neighbor is stealing your Wi-Fi, these two apps can help you identify devices using your connection and help you boot them off. 

Is your internet moving a little slower than usual? Are you seeing hints of devices you don't recognize in Windows Explorer, or when you cast media to your TV? If you suspect a neighbor is stealing your Wi-Fi, here's how to check (and boot them off).
"So someone's watching Netflix on my internet," you may say. "What's the big deal?" Even if you have a little bandwidth to spare, you probably don't want other people on your network, especially if it's unsecured. If someone has access to your network, they have access to all the computers on that network, and that's dangerous. They could access files you're unknowingly sharing, they could infect you with malware, and in certain situations they could even steal your passwords and other personal information.
As a result, you should take care to make sure each device connected to your network is one you can trust. Thankfully, there are free tools that'll help you see everyone on your Wi-Fi right now.

See Who's On Your Network:

Windows users can download a free, portable program called Wireless Network Watcher (scroll down to the Zip download link below "Feedback" to get it), and Mac users can download a free, slightly more complex program called Who Is On My WiFi from the Mac App Store. Both tools will provide a list of every device currently connected to your network, so you can identify the ones that belong to you.

Wireless Network Watcher:

To use Wireless Network Watcher, just launch the program, and it will immediately begin scanning your network. This will take a minute or two—you'll know it's working if the bottom-left corner reads "Scanning..." Once it's done, that message will disappear, and you'll be presented with a full list of connected devices.

Find Users:

The resulting list may look a little cryptic, especially if you aren't super tech-savvy, but don't worry. You can ignore the IP address and MAC address listings for now. If you're using Wireless Network Watcher, just focus on the "Device Name" and "Network Adapter Company" columns.
For example, I see an item named "Dulce" in Wireless Network Watcher, which is the name of my wife's MacBook. I see another with no name, but with "Philips Lighting BV" as the network adapter manufacturer, which means it's probably the hub for my Philips Hue lights. You can double-click on a device to add "User Text" that helps you identify each device, which will help you narrow down all the items in this list.

Who Is On My WiFi:

To use Who's On My WiFi, launch the program and choose "Yes, set up continuous automatic scanning" from the popup. Click Proceed on the next window, and the app will begin scanning your network for devices. You'll see the "Scanning" message in the upper-right-hand corner when it's working, so just give it a minute to do its thing.

Find Users:

If you're using Who Is On My WiFi, the "Description" column and the "Manufacturer" name that appears in the right pane when you click on an item is what you need. These two values will clue you in to what each device is.
In Who Is On My WiFi, you can't give custom names, but you can give the device a label like "Desktop" or "Tablet" and mark it as "Known." Go through the list and mark all the items that are familiar to you.

Get a Second Opinion:

If you're lucky, you'll be able to recognize all the items on that list, but there may be a few that don't have enough information. After going through my list, for example, I was left with a couple devices that listed no name and no manufacturer. However, I was able to get a little more information from my router's web interface.
Open your router's management page by typing its IP address in your browser's address bar. . Once there, look for an option that sounds like "Attached Devices" or "Client List." This will present you with a similar list as Wireless Network Watcher, but the information may be slightly different. After cross-referencing the unknown devices between the two, I found one of them was listed as "AzureWave Technology, Inc" in my router's interface, but not Wireless Network Watcher.

Eliminate Alternatives:

If you see any other unlabeled devices in the list, check around your house for any internet-connected gadgets you might have missed. I realized that my Amazon Echo wasn’t listed, so after checking the Alexa app on my phone, I was able to match its MAC address to one of the unlabeled items in Wireless Network Watcher.
If all goes well, you should be able to identify every device on your network. If there are any left over, and you’ve combed your house looking for other internet-connected devices and found nothing, there’s a chance someone nearby may be using your Wi-Fi.

Beef Up Your Network Security:

Even if you discover that a neighbor is stealing your Wi-Fi, you don't need to hunt them down and start a fuss—you can just kick them off with a change in router security. Head back to your router's web interface and find the option to change your password (usually under the "Wireless" section somewhere). If you don't have a password, you absolutely need to start using one, and it needs to be strong. Without a password, your personal information is up for grabs to any amateur hacker that drives by. Choose WPA2 for the password type, since it's far more difficult to crack than the now-outdated WEP.
If WPS is turned on, you should turn it off, since this feature makes it easier for people to crack your Wi-Fi password. (If you want to let guests on your Wi-Fi without giving them access to your devices and information, you can always enable your router's guest network.)
If you already had a password—maybe it was weak and easy for your neighbors to guess—changing it to something new should be sufficient to kick them off. Of course, you'll also have to re-authenticate all of your devices, but you should be able to rest a little easier knowing that all the devices on your network belong to you.