You would need to go on your router’s configuration interface and look at the client list. It should list some information including which Wi-Fi device is on which wireless network.
Unfortunately, interfaces are extremely varied, so I can only give general guidelines from the information I have.
Also, it’s important to note that you can’t run speed test or use the Internet on several devices concurrently if you want accurate results. That’s because your Internet connection is shared between all connected devices in your house.