For example, if there's a several part problem and the first problem isn't coming out, several questions being asked about it are in jeopardy. Write on the test that you know the first part isn't coming out and why you know it's not coming out right. For the rest of the questions, explain in words what you would do had the first part come out right. If an answer is blank, all credit is lost. If something is there, valuable points might be earned.
First intuition is generally right.
Think about all the times test answers were changed only to have discovered one was right in the first place.