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.
I agree, that uniqueness really is key, especially now, because there are so many coins in the market, sometimes having the same blockchain technology that supports them. Also, you need to check for those coins that have active developers that have objectives to help the coin further in the market such as increasing their market cap and circulating value. Another is if the coin commands support from a lot people, that would show you how reliable a coin is, and therefore, it could be safe to invest in it, at least safer compared to other not so valuable coins.