Expert skis are built for skiers who charge hard everywhere on the mountain—at high speeds, in variable snow, and on demanding terrain. They're noticeably stiffer and more responsive than advanced models, which means they reward precise technique with incredible power and precision, but they won't forgive sloppy turns or hesitant movements.
You're likely ready for expert skis if you comfortably ski black runs and beyond at speed, can hold a clean carve in firm conditions, and actively seek out steep, technical, or off-piste lines. If you find yourself wanting more stability and edge hold than your current skis provide at the speeds you're already comfortable maintaining, that's a good sign.
The biggest trap is buying expert skis because you ski aggressively a few times a season, but your technique isn't consistently there yet. A ski that's too demanding will fatigue you quickly and actually make you ski worse. Be honest about where your skills are right now—not where you hope they'll be—and consider an advanced or advanced-expert model if you're still building consistency.