The tried and tested Needs’ Analysis is always a good place to start.
This consists of ad hoc questions that will provide you with a clear picture of your student/class. You basically want to ask some questions to establish what your students’ expectations for the course are and mostly you want to get a good idea of their level of English.
For this it’s best to stay clear of self-assessment questions such as How well do you… or How do you rate your English and so on…as they are notoriously unreliable as most people find it very difficult to judge their own ability in a foreign language. Far better is for you to use a few open questions so that you can judge - objectively - their ability.
For more see this ICALwiki article