You can go to some professional digital currency assessment websites, and some experts have ratings for coins. My personal view is that if you want to invest in a coin, you must familiarise yourself with the project, understand the progress of this project and how the team members work.
By checking which coin is top listed or trust to the coin that many people are also trust believe to it, or study the coin you want before you buy that coin, also it can help you if you know someone have experience to that coin you want, so you have reference about it if its good coin or not. I suggest trust only to the coin which many people experience a good progress of their coin so you can also trust to it, or i suggest try to buy Ethereum this coin is one of the best altcoin, surely you will never regret to but it.