How do I design a test that truly validates a design?

Running a test is one skill. Designing one that truly proves a design is sound is another, and harder, because you have to know exactly what would count as failure before you start. Feeling that gap does not mean you are behind. It means you have reached the part that takes real judgment. Begin from the claim the design is making. What is it supposed to do, and under what conditions must it hold? A good validation does not just show it works once in the easy case, it pushes the design to the edges of what it promises, the heaviest load, the worst input, the condition people swore would never happen. You are not confirming it works, you are honestly trying to break it where it matters. Take one design and write down what it promises and what would prove that promise false. Design your test to chase that failure. Validation is the discipline of trying hard to be proven wrong, and being glad when the design survives it. Do that once well and the method carries to everything after.