There are sample tests for physics (I did GRE physics). So I guess you may find some for mathematics, just use our friend google :D. Just try to find test samples, and then after practicing a little, try to learn the issues in the exam that you're not so good at, for example, if you fail to solve an integral (on time or math basis), learn more about integration until you're capable of handling every possible integral that may face you in the exam in the time window you're give (1.5 minute for every question).
Good luck :)