In 2006 a group of scientists developed and validated a self-report mindfulness
measure that they called the Toronto Mindfulness Scale (TMS). Results of their studies showed that TMS scores increased following increased TMS scores increased with increasing mindfulness meditation experience and Decentering scores predicted improvements in clinical outcomes of mindfulness-based stress reduction programs. Thus, the TMS is a promising measure of the mindfulness state with good psychometric properties and predictive of treatment outcome.
With this handy app you can meditate for 15 minutes, give yourself the TMS questionnaire and save your Mindfulness scores (Decentering and Curiosity) to Healthkit. This data can help you gain an empirical, scientific insight into the effectiveness of your meditation practice.