Knowing the point where you pass good design and start over-engineering is an art developed through wisdom and experience. Sort of like the art of making good LOE estimates given incomplete requirements and an unknown team.

I would look to senior people who have boots-on-the-ground experience delivering and maintaining projects/products, ask them if they think you're overdoing it.

edit: I want to add, get input from non-engineers too. Ask them "in your experience, how far have projects diverged from the initial requirements/purpose? I'm trying to plan for the future but not over-complicate things"