Anet A6 stock power wires.. the live, the zero and the ground. Which is which
This picture shows the correct wiring for a British 13A mains plug:



The green/yellow wire must always be connected to the earth terminal (at the top of the plug). This is most important, and will prevent electrocution if something goes wrong.

The brown (live) wire should be connected to the fuse, and the blue wire to the remaining terminal. It is true that, since alternating current is being used, the device will likely work if the blue and brown wires are swapped, but it is better to stick with standard practices to avoid confusion.

I would not use a "test" screwdriver to determine which terminal is "live" in this instance, simply because it is unnecessary. The live terminal is always the one that the fuse is connected to. This is done for safety reasons since the device is automatically disconnected from the live side of the circuit if the fuse blows.

How to wire mains plugs is really beyond the scope of this stack. Watch appropriate YouTube videos if you want to make a competent job of it. The only advice that I will give is to always use a proper fuse of the correct rating for the device. Never use something like a paper-clip. Also, don't use the 13 Amp fuse that comes with the plug. The fuse is there to protect the cable, rather than the device. Most printers will draw less than 5 Amps, and will be supplied with a 5 Amp cable, so use a 5 Amp fuse.

Also, never work on a plug while it is connected to the power socket. This is most foolhardy, even if the socket appears to be switched off.