Simplest way to do what you asked in 2015:
from unittest.mock import patch
@patch('full.path.to.signals.question_posted.send')
def test_question_posted_signal_triggered(self, mock):
form = YourForm()
form.cleaned_data = {'name': 'Jan Nowak'}
form.save()
# Check that your signal was called.
self.assertTrue(mock.called)
# Check that your signal was called only once.
self.assertEqual(mock.call_count, 1)
# Do whatever else, like actually checking if your signal logic did well.
And with that, you just tested that your signal was properly triggered.