Having [students] do time writings without editing is an excellent way to assess fluency.
I’ve been having my students write for 5 minutes almost every class, usually at the beginning, sometimes at the end. Sometimes I set the topic, but most times I left them free to write whatever they wanted. I had them count the words and keep a record. These writing samples were great sources of information, both personal (about students) and linguistic (they revealed areas of grammar, syntax or vocabulary that needed more practice).
I did timed writings in almost all my classes, including “Speaking” and “Listening” classes.