Perhaps the most difficult area of copyright law is fair use.
Not only are the issues involved in fair use complicated and difficult to understand, but the answer is never satisfying. Since fair use can only be decided by a judge or jury and is handled on a case-by-case basis, there is no way to offer a definitive answer about what is or is not fair use. One can only determine the probability of the outcome based upon facts of the case.
However, the Copyright Website has a fair use visualizer that can help take some of the guesswork out of determining what is likely and unlikely to be considered fair use.
To use the visualizer, you must first have an idea about what kind of use you are planning. You then move the sliders to the appropriate positions to describe the use and check any relevant boxes. A bar graph at the top will tick up and down with each change, either inching up toward or away from the halfway mark, where a use becomes more likely to be fair than unfair.
Clearly this is just a rough estimate and should never be taken as legal advice. It also can’t take many variables into account such as the fifth fair use factor “Are you evil?”. However, it is great for a quick “second opinion” about the viability of a fair use argument in a case.
Please bear in mind that, as with all fair use claims, it will not prevent someone from suing you, it just might prevent them from collecting damages.