For some cases it's more obvious (e.g. dry skin tends to be flaky, oily might be, uh, shinier?), but when it's not to the extremes but also maybe not a mix, what's a decent rule of thumb to narrow it down?

Similar regarding sensitivity. A lot of skincare products market towards these conditions with the expectation that you'll know somehow, but it's not always so obvious, and a doctor's visit isn't always affordable (which is probably the best answer here, tbh, but let's suppose it's not a severe skin condition one's trying to identify).