I like to have my students write a diagnostic essay the first day or two of class. This is an ungraded assignment that they complete in class. This way I know it is truly what they know and not what their parents, sibling, or friend knows. I keep the prompts generic- and typically have my students respond to a famous quote.
I don't include a length requirement but do use the term "essay." It is always interesting to me to see how much a student chooses to write. I find these essays quite informative.
For teachers in the 7-12 realm, you may want to be able to follow up for data purposes and give a similar prompt around winter break and another at the end of the year.
Would you be interested in a ready made set of 5 diagnostic essay prompts with lined paper? I created a set on Teachers Pay Teachers- with 5 prompts you can choose to give multiple ones to one class, use different ones different times during the year, or give each class period a different one (I know reading the same thing over and over can get boring!). It's really a steal and can be printed or projected easily! I also provided a checklist that can be used for simple scoring.
Thanks for Reading!