I ask all of my students to review Twinkles with me. I have reminded them that we will do this during their lessons this past week. For some of my young students, that is what we are working on anyway, but for my older students, this will involve a revisit down Twinkle Lane.
I listen for all sorts of things in my students' sound and playing depending on the level of maturity and advancement. For example, I would expect much more sophisticated use of the bow in a book 6 student than a book 1 student.
Here is an article I wrote previously about the types of things that I look for when playing Twinkles: Twinkle Points.
Other possible challenges for more advanced students could include using vibrato, playing in higher positions, transposing the song into different keys, starting up bow, playing spiccato or sautillé.
For younger students who can already play the Twinkle Variations straight through, I look to see that the main teaching points are still in place. If a book 1 student is getting near to book 2, then I watch that the student is beginning to incorporate more bow usage. If the student can play each of the Twinkle Variations individually but not as a whole, then I work to see if we can play through the first two or three variations without a break. Eventually my first mini-goal will be to play through variations A-C without stopping, but this may take a few lessons before the student has built up the stamina necessary to do this. Once the student has passed the variations A-C hurdle, I generally find that the student can then play through all the Twinkle Variations without problems.
Also for my book 1 violin students, I teach them how to play the variations along with the piano accompaniment. This is difficult for some students and takes a few lessons to master.
Let me know about some of your favorite ways to practice, review, or use the Twinkle Variations.