process
Hallway testing
Informal usability testing with whoever's nearby – watch a real person attempt a real task, and stay silent when they get stuck.
Also “guerrilla testing”: grab a colleague from another team, hand them the design, give them one task, and watch. Deliberately cheap, and backed by real maths – see the 5-users rule for why a handful of informal sessions surfaces most of what a formal study would.
The one discipline that makes or breaks it: stay silent while they’re stuck. The instinct to jump in and explain destroys the entire signal – the hesitation is the finding.