[RhysU]

Ideally, the slow code is available alongside the fast code and the unit tests confirm they are sufficiently equivalent. If that's not possible, the go-fast bits need to be refactored until it is possible. Don't forget that error handling is part of the equivalency testing.

[DylanSp]

Beyond unit tests, this seems like a great opportunity to leverage property-based testing, at least for pure functions; checking that both versions return the same results is an easy property to think of.