Gut instinct. If you’re a working tester you probably use it, oooh, roughly, I’d say, well, on average, y’know, in a manner of speaking, perhaps, erm, just about all the time.
Feel, or the intuition bred of experience, is rolled into everything you do. In every decision you make, it’s playing a part. Every time you speculatively poke the system this way or that, or don’t poke it, or poke it twice quickly, or with a series of sticks with lengths in powers of two, or with a null stick, or a log, a twig, a stake or a bâton, every one of those, and pretty much everything else, in some way, involves your gut.
Which is why you’d better feed it. And what does it like to eat? Data.