Actually easy way to know when we should sell the coin we buy, when you think the profit is sufficient then that's the right time to sell it. In addition we should often read and be active in discussion forums such as bitcoin discussion, trading discussion, or it could be in local boards. From there we can get knowledge about how to be a good trader and also about the news about bitcoin and altcoin, so we can better know when we should buy and sell our coin so as not to cutloss and always profit.