Well, it's not that hard to know, all you need to do is take a project and evaluate it from top to bottom, start from their website, read the whitepaper, check their docs and resources, their social engagement rates, the interactions of the public with them or about them, and the influence and interest of other big names or projects if there is any.

Once you are done with that, you will pretty clearly see if the project has got any potential or if it is just empty from the inside and is only here for a short while. Now do that with every project that you want to know if will exist in the future or not.