How do you tell if a detailer actually knows what they're doing

[ricardobravo]

Asking because I've had two experiences now, one great and one that left buffer marks on my boot lid, and I can't figure out what distinguished them before I handed over the keys. Both had good reviews, both seemed professional on the phone, both talked a good game about paint correction and machine polishing. But one clearly knew what they were doing and the other clearly didn't. I'm now looking for expert car detailing in Khalifa City for my other car and I want to avoid a repeat of the bad experience. What questions actually reveal whether someone is competent? Do you ask about products, technique, training? Do you look at their workspace? I need a better method than just hoping this time.