Ian Shepherd does plenty of great work on educating people about dynamics and loudness for music, and he has collaborated with MeterPlugs on a website called Loudness Penalty that analyses your music’s loudness to see how much it might get turned down when normalised by the various online platforms such as YouTube, Spotify, etc.
Ian has put together a short video in a blog post explaining the website and how to use it at:
http://productionadvice.co.uk/loudness-penalty/
And you can access the Loudness Penalty website directly at:
http://www.loudnesspenalty.com/