How to determine if a MAC address is trustworthy

I've downloaded WiFi Guard, and it shows my PC and ONE of my TWO routers by vendor name, but it shows four IP and MAC addresses - two other devices. One says Azureware, which is probably my Chromecast attached to the TV downstairs by coaxial input and HDMI output, but that still leaves the fourth device without a designation, only an IP and MAC address. Since I have one PC, one modem, two routers and one VOIP, I'm paranoid as to whether the other device is my router, my VOIP or a neighbor or what.

My connections are: From Modem by ethernet to VOIP, then VOIP by ethernet to WiFi Router 1, then from WiFi Router 1 port by ethernet to upstairs computer and also from 2nd WiFi Router 1 port by ethernet to downstairs WiFI Router 2, then by ethernet to the downstairs computer. How can I determine if all four devices on WiFi Guard are legitimate or if one of them is a neighbor stealing my WiFi? I do have WPA2 on both routers and passwords to allow devices to join.

Thanks, if you can make sense of this and be of help.

Unfortunately there is no easy way to tell what it is.

It could still be your device or your friend's. The simplest way to identify the devices would be to think what in your place may have access to the Internet. It could be gaming consoles, a smart TV, mobile phone, etc. Turning various devices off and on one by one will help to find out what is what.

If you still have no idea what those devices are, it makes sense to change your WiFi password in your router to stop them using your network.

You may also want to check the beginner's guide.