They determine what tests based on what the suspected issues are. A kid with language/writing issues might be tested by the speech therapist, the OT (for fine motor), and a reading specialist. A kid with problems sitting still will get a different battery of tests, including attention and gross motor.
You can ask them what tests they will use as they start the evaluation (these tests generally require consent, so you will have to sign papers and have an opportunity to ask questions), but it will be tailored to what has been observed about that child.
DeeDee