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."