Whether an answer key is available for a math test depends on if the teacher created it themselves, but there are places, such as StudyBlue, that allow users to freely post study guides. If the teacher is using a generic exam, then a student may have a chance at using StudyBlue to help find answers.
StudyBlue is a tool for studying for exams. Students post review questions and answers for their peers to see and use. Some answers may appear on generic exams taken by the students.
However, a customized exam created by the teacher will not have an answer key available online.