I don't think it's for you to tell; I think it's like (as someone else mentioned) dancing or playing guitar or other creative arts - the only judge is other's f

I don't think it's for you to tell; I think it's like (as someone else mentioned) dancing or playing guitar or other creative arts - the only judge is other's feedback on the output you produce, preferably some kind of balanced mix of experts in the field, peers at your level, and the public at large. We don't really have standard competitions (no, HackerRank and Kaggle don't quite count) but certainly (much like creators) coding in the open, collaborating, and soliciting feedback on the things you've code is probably the most certain way. One thing that I've always admired about 30k hours high performing folks is their sheer level of output; because they have mastered things, producing new things becomes something almost effortless, and so their challenge becomes keeping psychologically grounded in that high performing zone (avoiding the "yips"); collaborating with others to break new ground; and putting their talents to their most effective use.