How can I spy on a nested dependency in with sinon

I am really new to mocha/chai/sinon, and testing in general. I've managed to successfully test a basic express server, a function returning promises, and a basic sequelize setup to get my nose wet; but I am stuck on spies/stubs/mocks.

My fist hiccup is trying to check that glob has been called in an external module:

//in utils.js
var glob = require('glob');

module.exports = {
  funToTest: function (msg, callback) {
    console.log(msg);
    glob('*md', {
      cwd: 'files/'
    }, function (err, files) {
      console.log(files);
    });
    callback();
    callback();
  }
};

Using a mocha/chai/sinon/sinon-chai combination:

// in utils-test.js
var utils = require('utils.js');
var glob = require('glob');

describe('Utils', function () {
  describe('funToTest method', function () {
    const callback = sinon.spy();
    const globSpy = sinon.spy(glob);

    before(function (done) {
      utils.funToTest('Files:', callback);
      done();
    });

    // This PASSES fine
    it ('should call our callback twice', function () {
      expect(callback).to.have.been.calledTwice;
    });

    // This NOT SO MUCH
    it('should call glob once', function () {
      expect(globSpy).to.have.been.calledOnce;
    });
  )};
)};

The above fails with an assertion error:

AssertionError: expected glob to have been called exactly once, but it was called 0 times

So how do I spy on the glob dependency in utils.funToTest to see if gets called?

Thanks as ever for any pointers...