You know which altcoin to invest in when people are talking about it a lot. For example, BCU. Although there's not that much actual supporter behind it but mostly whales that invested millions for the coin, if you've heard that BCU is about to go out of exchanger you'll know that it will increase in price because everyone's talking about it on twitter. Even though most people are talking how bitcoin will never have fork or whatever, it still caused hyped to the BCU coin.
In altcoins in general, people say that they actually analyze the chart. I don't know the complete guide to saying "Oh, this coin is gonna be poppin'." But hype is the only thing that increases the price most of the time and not the altcoin's usability or utility.
In fact, I don't think anyone even knows when are altcoins going to rise. Some people are just able to determine that the coin will rise eventually, but there's no time frame. Others set a price and then sell all of their coins at that price all at once. While others set different sell orders like 1/4 for 1000 satoshi, 1/4 for 1300, and then the remaining 1/2 for 1500 satoshis.