Read their whitepaper!
Yeah. That simple. You look at how unique their project is and how the community would benefit from it. Then next is the team behind it which is really important if they're legit or not. And you can add a little bit of extra research about it.
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a good coin is a coin that has an ever-increasing development and that is able to have a high level of interest. we can see on the coin market cap for the development of each coin to know the development of what the coin that increased and very capable for reliable future.