Understanding functionality of new zigbee devices and creating converters for them

[wifiuk]

I have just added a PR for a smart plug and I can turn it off and on etc.

but how do I get it to tell zigbee2mqtt its a wired power source.

One thing I don't understand and would like to understand is when you get a new zigbee device, how do you interrogate it to know what features it has other than adding on and off and how do people convert this into the converters, are there any really good guides for this?

So I might be missing functionality because I don't know how to interrogate the device correctly to write the converters.