In a command line, navigate to the folder which contains your project.godot file.
Then, still via that command prompt, launch the Godot executable with the '-v' parameter.
If executed correctly, Godot will automatically start running your game (regardless of whether you have an Editor instance open) and it will include debug output. This includes a moderate amount of information about leaked resources on exit.
It won't tell you where the leak occurred but it will tell you what's leaking.