First thing that you need to do is to read their whitepaper to know what is the concept of their project then you check their timeline so you will know what should be the progress and what are their project plan also to know when will the project ends. Check their members especially the bounty manager so that you will know what is the rank of the members and to know if the bounty manager has done a previews ICO.
Reading the white paper is useless, you still need to do it but it is impossible to determine if a coin is going to be successful just with the white paper alone and there are many reasons for that, scammers will copy ideas of legitimate developers just to make their projects seem more real, another problem is that the white paper is just an approximation of what they are trying to achieve but when faced with the problems of real development it's not rare for the white paper to be disregarded.