A good thesis statement will focus on a specific aspect of a given topic, inform the reader's interpretation of the paper and make a claim that is open to dispute. It will usually appear as a sentence in the opening to the paper.
In order to decide upon a thesis statement, it is first necessary to thoroughly research the relevant existing literature. It is important, more specifically, to look for relationships and contrasts between the known facts about a given topic. A good thesis statement might take such a relationship and explore its significance within the wider context of the topic.
Instructors and tutors can be consulted for their evaluation of a working thesis statement, but there are some general criteria to be aware of. Chiefly, the thesis statement must directly address the topic or question provided. It should also invite discussion by making an argument. If the topic is the Civil War, for instance, a good thesis statement might compare or contrast the opposing forces' divergent reasons for fighting each other, rather than simply state that their reasons were sometimes shared and sometimes different.
Making the thesis statement specific is important to focus the paper, but it should always be related back to the broader context.