In general, online aptitude tests are most common. What elements are tested varies. Most tests focus on numerical and logical/diagrammatic reasoning, others also on verbal reasoning.
You can prepare for the assessment quite well. A sample/practice test is mostly provided, which is very insightful. You can also prepare assessments by using Wim Bloemers practicebook: ‘De nieuwe assessmentgids’. Furthermore, you can find various online practice and training websites, both paid and for free. Just make sure you know upfront which elements will be tested.