Depends how you do it. Having a sane communication model/interface between the services allows you to test it in two ways: 1) You test the internal structure with unit tests 2) You test changes to the API contract with E2E tests Both are fairly straightforward once you set it up properly.

Same goes for the monolith, assuming you do your work on following somethink like the SOLID principle, it is easy to test piece by piece. The only thing here is that the interface contracts are internal to the codebase vs external to other codebases. (You might argue the last point actually forces you to write a more coherent API contract in both situations)