I’m watching your playing. Maybe at a gig, in orchestra and of course during your violin lesson. It’s like x-ray vision, though not in a creepy sort of way. Call it an occupational hazard of being a long time violin teacher.
I don’t have to hear you play. A very few visual cues will tell me whether you approach the violin with ease and mastery. Or if you are engaging in battles (or just skirmishes) with your instrument.
Perhaps the biggest canary in my coal mine is your left hand, and how you use it to produce vibrato. Because a great violin vibrato is the natural result of good basic playing habits.
For my fellow vibrato nerds:
There are only a very few simple motions needed to produce vibrato, I describe them in obsessive detail in this video.