Jasmine: How to test if private function of nodejs module has been called

I'm writing nodejs module for learning purposes (logging service) and I have problem with testing part of the code.

Shortened version of code I want to test is (whole project HERE):

var tupelo = exports;

var logMessage = function(debugLevel, color, message) {
    var time = new Date().toISOString();

    console.log(
        color(debugLevel + ' on ' + time + '|  ' + message)
    );
}

tupelo.verbose = function(message) {
    if (levelEnabled('VERBOSE')) {
        logMessage('VERBOSE', colors.gray, message);
    }
}

I'd like to test if logMessage was called based on return value of levelEnabled('VERBOSE') (true / false)

Because it is a private method, I found that I can access it from my spec with rewire

var rewire = require('rewire');
var tupelo = rewire('../src/tupelo');

describe(' displays logs only on enabled levels', function() {
    it('should display all logs below or equal to debugLevel', function() {

        var logMessage = tupelo.__get__('logMessage');

        spyOn(tupelo, 'logMessage');
        debugLevel = 'VERBOSE';
        tupelo.verbose('testing')
        expect(tupelo.logMessage).toHaveBeenCalled();

    });

});

Except it doesn't work. Error jasmine returns is:

Error: logMessage() method does not exist

I'm obviously not aware of something that I probably should be. How can I properly test that? What am I missing?