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.
Or you can check its market value and its chart history, you can easily track its progress and the volume of its market. I also usually check its trade volume from different platforms including its past months just to see if its market is circulating well and being invested on by a lot of people.