When I overhear random people on the train, my coworkers during coffee break etc. discuss bitcoin, I know that the bubble is about to burst. This predictor has never failed.
http://archive.fortune.com/magazines/fortune/fortune_archive...
The 16 year old brother of a friend of mine suddenly wanted to buy Ethereum but didn't know what it was. A month before that he wanted to get rich with binary option websites.
I sold him the amount he wanted (30 eur worth at around 380USD/ETH) and then sold all of my ETH.