Knowing the project very well,it's purpose and roadmap. Read the white paper and better to know the people holding the project. Another thing I consider if they are active in telegram group,they are answering your question quickly and also my instinct which i felt that the ICO is good and become successful.  But you know sometimes even we know that the ICO project is good to invest but at the end the are failed. Maybe we can't avoid it because i believe the success of every ICO is not just depend on the project itself but it depends on the acceptance of the community which is the demand.