A thesis is a declarative sentence, never a question. “Who was the best US President?” is not a thesis because it is a question. “Washington was the best US President” is a thesis providing the word “best” is precisely defined. If “best” is not precisely defined, then this is not a thesis because “best” is vague.
A thesis states an opinion which can be defended or countered. “Laws should prevent children younger than 18 from marrying.” This is a thesis. It is an opinion which can be supported by evidence. It can be objected to with other evidence.
A thesis is not a statement of facts which can be verified. “Washington is the only President to be elected unanimously” is not a thesis because research shows this is a factual statement.
A thesis is not an opinion of personal taste. “I need to attend college” is not a thesis. It cannot be researched scientifically.
A thesis is stated positively, not negatively. “Washington was not the best US President” is not a thesis.
A thesis does not use biased or untruthful language. “Dangerous hand guns should not be sold” is not a thesis. “Dangerous” is a biased word.
A thesis uses precise language. “Some amphibians should be put on the endangered species list” is not a thesis. “Some” is not precise.
A thesis is about one idea, but that one idea can be subdivided. “The US Civil War had two causes: slavery and states’ rights” is a thesis. This thesis is about the causes of the Civil War (one idea). “Roses are easier to grow than irises but harder to grow than day lilies” is not a thesis. It contains two separate ideas.
A thesis is researchable using scientific evidence or the scientific method. “Nothing escapes black holes” is not a thesis. Scientific research has proven this statement to be false. “More two-year-olds today are fat than in Boston in 1776” is not a thesis because it is impossible to research how many two-year-olds were fat in Boston in 1776.
A thesis deals with real, not conditional or hypothetical information. “If Elvis were alive today he would be a billionaire” is not a thesis because Elvis is not alive today.