I've often found that not being able to test something in a sense of "given: input, expect: output" is a smell that you're taking the wrong approach and that a

I've often found that not being able to test something in a sense of "given: input, expect: output" is a smell that you're taking the wrong approach and that a better factoring for your code exists (this obviously isn't always the case, perhaps the most common exception being testing integration with third-party code).