I moved my washing machine recently and have to add to the ground cable.

I bought a copper wire and twisted the wire around a few times then bashed it with a hammer. It doesn't feel right. How would I test it?

I presume I need to buy a multimeter or is it another device?

Can you not replace the wire so you do not have a joint in it?

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Everything in my roof space is twisted wires and taped. I would forget the hammer bit, or replace the wire or buy a connector.

Connectors are really poor here in Thailand.

Loose connections are really hazardous.

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Well, this is a ground wire so if it's well twisted you don't need tape or a connector. Keep the hammer in the box - you might break the wire doing that. To test, just give a good pull and if it doesn't come apart, you're good.

RE "twist and tape": I'll say it again... from what I've seen from most Thai sparks they twist and tape a connection that is almost impossible to take apart. That's probably better than using the crap connectors available here but whatever is used - making sure the connection is secure is a must.