If your child is age 9 or over then a standardised handwriting assessment can be carried out. Depending on the outcome a perceptual assessment may also be advisable.
I would also get an overview of their fine motor skills and hand function prior to handwriting. As well as any issues they may have with concentration/ focus, ability to sit still and the physicality of writing. Alternative writing equipment may well also be explored.
If your child is below the age of 9 the above still applies but basic shape construction and execution would be investigated; as well as any equipment that would aid their handwriting.
Dysgraphia is a specific learning disability that affects how easily children acquire written language and how well they use written language to express their thoughts. It can present itself as difficulties with spelling, poor handwriting and trouble putting thoughts on paper. Although it can occur alone, it can also be found alongside Dyslexia, Dypraxia or ADHD.
Writing consists of a mixture of motor skills and information processing, requiring both the ability to form ideas in the mind and the ability to get the muscles in the hand to put those ideas onto paper. Generally people with Dygraphia struggle to write quickly and legibly, and place greater demands on their working memory whilst trying to write.
Messy handwriting alone is not necessarily a sign of Dysgraphia, symptoms will also include inconsistences in writing e.g. mixtures of printing and cursive writing, upper and lower case, or irregular sizes, shapes, or slant of letters; tight, awkward pencil grip and body position; unfinished words/letters or missing words; avoiding writing or drawing tasks; tiring quickly while writing; saying words out loud whilst writing; having their face close to the page; irregular spacing between words and letters or irregular positioning of words between the lines or margins.