Hi Sana,

At the moment there is no practice test that you can do. Odoo does not provide this and there is no information about the exams itself so the only way to find out is to do the exam itself.

As for preparation: there is quite a lot you can do. The best place is the Odoo documentation (https://www.odoo.com/nl_NL/page/docs ) as this contains user guides that are written by Odoo. This also means this is how Odoo wants you to do it and means you're sure that what you learn is according to their plans. 
If you really want to learn a lot of functional stuff and don't mind paying a bit for it (125€/month) you could also have a look at the official training platform which you can find at https://odoo.thinkific.com/courses/odoo-functional.
The rest of the knowledge can be acquired by 'playing' with Odoo and getting hands-on experience or by searching online tutorials.

Regards,
Yenthe