How can I see who is connected to my wi-fi network?

The first method is open terminal & type netstat -a : active internet connections including servers are listed & if you want to verify any suspicious device is connected to your network . Please open airport utility directly from spotlight , keep on holding option key > double click on base station . A list of devices will be shown .

Verify the hardware addresses of the devices that you are using .

Also , type system profiler in spotlight > click to network > BSD device name , confirmed interface name , interface name should be always en0 . it should be never en1.

You should not have anyone using your network (unless they are part of your household) - do you not have a strong password to protect it?

What is the make & model of your wireless router that is producing your Wi-Fi network?

You mean to say what devices apart from yours are connected to your network please explain ?

I use the app "fing" on my iPhone to check who's connected to wireless networks.