Perhaps the hardest part of "coding" is identifying skills. Sometimes it's pretty straight forward, but other times skills seems to go by too quickly to identify the minor specifics which often make all the difference.
The most basic example is telling the difference between tucks, pikes, and layouts. Each are away that you can perform a variety of skills, from tkachevs to double backs. (These pictures do NOT have perfect form, but you still get the idea.)
There's also a variety of leaps to know about, here are a few...
The hardest leaps to distinguish in my opinion are Split Leaps & Switch Leaps. In a switch leap (or a switch ring leap), the front leg swings forward, past 45 degrees and then swings behind to finish the leap. Bailie's left leg is green and her right leg is pink. This happens in less than a second, so it's much harder to tell in action.
Although I'm no master here, I learned a lot by watching code videos by MostepanovaFan on YouTube. There are videos that will tell you the difficulty and the name of virtually every skill in the COP. To be honest, the key to quickly recognizing skills is watching a lot of gymnastics.
Pictures from Flip Images, Comaneci Salto, Grace Chui & USA Gymnastics