How do I tell careful engineering from over-engineering?

You design for robustness, and now you are learning the subtle line where sound rigor turns into anxious over-engineering. That is a sophisticated thing to be wrestling with. It means you are good enough to build in real margin, and honest enough to ask when the margin has stopped serving the design and started serving your worry. The difference is usually in what is driving the choice. Rigor answers a real risk with a proportionate response. Over-engineering answers a fear with more than the risk warrants, piling on safety factor to quiet an anxiety rather than to meet an actual failure mode. The work looks similar from outside. The difference is whether you can name the specific risk each measure addresses. So make that the test. For each safety factor or reinforcement, ask what precise failure it guards against and whether the added margin matches the real likelihood and cost. If you can name the risk, it is rigor. If the honest answer is just in case I missed something, that is worth a closer look. That single question keeps your discipline sharp and your anxiety from quietly designing for you.