How do I validate AI simulation and design output?

You have been misled by output you did not fully validate, and that lesson is worth more than any tool. A simulation is a confident story about how something will behave, and confidence is not correctness. The polish of the result is exactly what tempts you to skip the checking. The machine does not know if your inputs, boundary conditions, or assumptions match reality. Feed it something slightly wrong and it will hand you a clean, plausible, wrong answer with no warning. Validation is your job, and it is not optional: sanity-check against known cases, hand calculations, physical intuition, and the simple question of whether the result could possibly be true. If it violates something you know in your gut, believe your gut and dig in. Make one rule: no result ships until you have checked it against something outside the tool. Before your next output goes out, run a rough hand estimate or compare it to a case you already understand. If they disagree, you have found either an error or something worth learning. Either way, you have done your actual job.