Certification exams have pretty high stakes. How do you know when you're ready to sit for the exam? You can purchase the official textbook if there is one available, or you can try Skillset. It is a free online assessment platform that can be used as an exam prep tool for those working toward a professional certification. Currently the start-up only offers exam prep for IT certifications and the Project Management Professional (PMP) certification, but it's in the process of developing assessments for accounting and customer service management certifications, as well as for employment screening.