The first thing that makes me raise an alarm that an ICO is scam is when the idea is shit and not feasible at all. Also, sometimes the site they have, the ICO platform, is a sign if the project is scam or not. If the site hosting is very cheap, there are chances that the project is a scam. Lastly, if the ICO dev lacks public relation, it will later on result into a PR problem and might eventually scam people.