GbxDump allows you to view the header data of Gamebox files.
For example, you can use it to check whether all the locators (for signs, music or a mod) are set correctly.
Simply load the map in GbxDump and check if there are any custom <dep> entries that don't have a url property - that means they will not be loaded correctly for everyone else! Hint: You can ignore all the entries you don't recognize by the filename - there are a few entries for default signs/images/colorgrading that don't require a link.
See the example screenshot below for reference.
More info here: https://github.com/Electron-x/GbxDump