There are a few signs...
Their goal, token amount, token value should be corresponding with each other, if there's a fault in that math, that's a red flag for me.
Their team members and group of advisors; if there are any of them that was realted in a scam ICO before, that's another red flag for me.
Their roadmap should be realistic; sometimes a team may not be able to deliver roadmap instantly and it may take time to see a roadmap but it should be out before the ICO, if it's not that's another red flag as well.
And last but not least; I always keep this in my mind: "If it's too good to be true, it's probably not."
very good thoughts! Since most of ICOs start with white papers. Personally, I would read through the paper to see if the overall quality looks good and specifically, the presented use cases sound reasonable to be put in reality. Then I will search the team members' linkedin profiles and advisors. if most have good records and many credited connections, I would give more credit to the ICO. The last but not the least, a reputable partner will make big difference.