Use an IDE like IntelliJ IDEA to decompile the classes and see what the code is actually doing.
If you are looking to analyze or report on the contents and safety of this specific JAR file, you can use several standard Java development and security tools: Ways to Analyze the JAR File FinerDupes v2.jar
Knowing the source (e.g., a specific GitHub repo or a Minecraft modding forum) would allow for a more detailed functional report. Use an IDE like IntelliJ IDEA to decompile