It depends on the number of questions in the test and the amount of options per question. If a question has n answering options, then the probability of randomly guessing the right answer to the question is (1/n), assuming that while guessing you assign the same probability to each option. If the test has i questions, the probability of acing the test (answering each question correctly) becomes (1/n)^i
For an example, if you have a test with 10 questions, each with 4 answer options, the probability of acing it by randomly guessing is (1/4)^10 = 0.00095%. Basically impossible.
Notice that the probability decreases with both the number of questions and the number of options. The longer the test and the more answer options per question, the harder it becomes to pass by randomly guessing.