Jolly Phonics teaches reading and writing through 42 letter sounds, each with a sound, an action and a letter formation. Assessment is one-to-one and oral - a teacher sits with a child, listens to them produce a sound, and watches them form a letter.
That is the right way to teach it, and an impossible way to measure it across a class. A teacher with thirty Early Years children cannot listen to each of them produce 42 sounds with enough attention to catch which phonemes are consistently wrong. By the time a pattern is noticed, months of practice have set it.
Handwriting carries the same problem in visual form. A teacher can see that letters are untidy without being able to say whether the cause is baseline drift, inconsistent stroke, or a letter formation that will later block cursive entirely.