A Hello! This is an excellent question and you are very smart to do your research on what makes a good filler injector beforehand. Regardless of whether the injector is a doctor, surgeon, nurse, nurse practitioner, or physician assistant, it is best to go to someone who does filler all day everyday. Filler is a game of practice, and the more you do, the better you are at it.
Try and get a feel for how your provider spends his/her day. Ask yourself these questions: Does the provider's website list 20-30 other procedures? If so, this might not be the best choice because they are busy with non-filler procedures. Is this a surgical practice? If so, the doctor may be in the operating room most days. Just because someone is excellent with a scalpel does not necessarily mean they are excellent with a filler needle. What procedures do you see most often on their Instagram feed and Instagram Highlights?
I will say nurses/nurse practitioners are doing the majority of the filler procedures out there; in fact every single major filler manufacturer (Allergan, Merz, Galderma) all use nurses/NPs as their national trainers, meaning they are the ones training doctors and surgeons.
All in all, every provider knows what procedure they are best at, however, you might just have to do a little bit of probing/research to figure out.