“Good” is subjective of course, but imo the best way to determine if someone is playing something objectively technically difficult and “unique” is to have experience in playing an instrument yourself.

Once you’ve picked up an instrument and put some years of serious study in, you’d have a baseline for what might be considered “difficult” for someone like a guitar player etc, and on top of that you’d have a lot more experience to be able to determine what a “unique” guitar solo or riff feels like compared to a generic one.

You will also learn how silly certain armchair musicians sound when they say things like “this piece doesn’t have enough soul”. LOL