It should be enough that the .generated.h file has an older timestamp than the .h file, or doesn’t exist at all. You can try deleting it and building and see it gets built.
If you find and open the .uhtmanifest file for your project+config then it will tell you which headers are being considered for being rebuilt. The file is in compressed JSON format - you should be able to search for the name of your header in any normal text editor, but you might want to use a JSON formatter if you actually want to read it.
Steve