By using one's brain. Understanding the code. Knowing what effects what. And by running it, particularly the changed flows. The disadvantages to tests/TDD is (a) it assumes the developer is stupid, and (b) even if he is not, he has the additional burden of creating and updating the tests as he goes along, slowing him down. Yes there can be downsides to not having tests/TDD. But the upside can vastly outweigh the downside, in many situations. YMMV.