Measuring and determining cassette wear is not an easy task. Basically the best advise I would know to give someone is “it comes with lots of experience”. I have seen cassettes changed when they still looked practically brand new and wondered why anyone would throw their money away doing this.
HERE is an article about this subject.
Here is a video showing how to use a wear indicator tool.
Here is a video showing how to determine wear of the chain, chainrings, and cassettes.