We don't know how

I don't know how to test is really the only acceptable answer, assuming it is quickly followed by, but I'm willing to learn. We're developers. We're the experts in the room. We're paid to know the answers. It's scary to admit that we don't know something. It's even scarier to start something new. Rest assured, it will be OK. Once you get the hang of TDD, you’ll wonder how you managed before. You'll refer to those times as the dark ages, before the discovery of the wheel.