If you're using Electron 1.8 or later, you can specify the --inspect-electron flag when running electron-forge start, which will set the Electron --inspectflag with the default debugger port.
electron-forge start --inspect-electron
This will allow you to open chrome://inspect in Google Chrome and attach a debugger to the main process of your app.
Debugging with VS Code
Debugging your Electron main process through VS Code is ridiculously easy with Forge. Simply add this as a launch config in VSCode and you're good to go.
You need to be using Electron 1.8 or later for this launch config to work.
If you are using < 1.8 you should really be updating Electron anyway.