For me to recognize the best time for selling a coin was being at its peak. For example, when the coin you were holding was being at the high ends just like like the dencoin tokens then thats the right time for you to sell. I would also say that its better to buy coins at a lower price when the prices are down, so just hold and sell them when the prices increase at the market. So in this kind of case, the more coins you can sell is tje more profit you can get.