How to know if bable/webpack initial compilation finished

Question

I am making my first bable plugin that transforms codes during the development.

This plugin will be used with webpack and should handle multiple files.

I need a way to know when babel or webpack has finished the first compilation and starts the watch mode, so at this point, it transforms code after file changed.

Something like this:

export default function ({types: t}) {
  return {
    visitor: {
      Identifier(path) {
        if(COMPILED_SUCCESSFULLY)
        {
           now transform code if file changed.
        }
      }
    }
  };
}

Looking at documentation there is no event, variable, or function that tells if compilation finished, but surely there must be one! any idea?