If you are using JetBrain IntelliJ IDEA then there's another possibility: "workspace.xml" file under the ".idea" folder, in your project's root folder.

Open the "workspace.xml" file, find the text "SPRING_BOOT_MAIN_CLASS" (all UPPER CASE with under line/score between the words). It should be the text value for the "name" attribute of a option tag. The "value" attribute of that same option tag should be your package.name.and.main.class.name of your project.