Depending on the assignment settings your instructor has chosen, you may see a "Check Answers" button on some or all questions on your assignment. Frequently it will be set to show on all questions except multiple choice.
Your instructor may set a limit on how many times you can use it; once you reach that limit (which happens when you use the button when it says "You can check your answer 1 more time before the question is locked") you will no longer be able to change your answer.
When locked it looks like this:
The question options are grayed out, and you can no longer change your answers.
The "Check Answers" button only checks the answers that you have already entered; on a multi-part question, it will only check the answers already entered. Other parts of the question will be left blank.
Note: the "Check Answer" button will NOT always tell you if you are going to get full points on a question.
On multiple-selection questions, the "Check Answers" button only shows you if the answers you have selected are correct. The "Check Answers" button does not tell you if you have not selected all the correct answers, so using the button does not guarantee that you will end up with full credit on the question. For example, in the following image, a question asks which numbers are odd, which are even, and which are prime, and lists the numbers 2, 4, and 3. The even and odd answers have been selected correctly, and the check answers button confirms this, but no selections have been made in the "Prime" column.
Upon submission, this question would not receive full points, because both 2 and 3 are prime numbers, and those answers have not been selected.