Currently, when adding a module into a custom report, once the module is added the orange plus sign turns into a red X sign, and that's how you can differentiate the modules which have been added, and the ones that haven't, but red and orange are very similar, so it's not very obvious which modules you have added, and so you need to really pay attention so that you don't miss the ones you have added.

It would be a lot easier if the added ones were blue for example.