There are no formulas you can use to figure out if something is fair use, no clear-cut test you can apply. This can be frustrating when you have bigger things to worry about - like your thesis! It can be helpful to remember that fair use is, essentially, the part of the copyright law that protects our First Amendment speech rights and that its strength lies in that very lack of specificity.
So, how do you figure out if your use is fair? You look at:
These are called the "four factors" of fair use. See Columbia University's copyright website for more detailed information.
A fair use checklist is a tool you can use to help you decide if your use is fair.
A fair use checklist is a decision-making tool; in the end, you still have to make your own determination. What the checklist can do is let you know what kinds of things favor fair use and what kinds of things oppose it, giving you a framework for thinking about your use of a work. And, if you print it out and save it, it can provide documentation of your analysis (which can be helpful if anyone questions your decision).
Need help figuring out if your use is fair? Contact a librarian. We can't give you legal advice, but we are happy to talk it through with you and share our thoughts!