Choose an ICO whose tech you can understand. If the philosophy and mission of the ICO makes sense to you, then that's one green sign.

Then look at the team of the ICO, do they have many years experience? Are they well-known people? IF you can answer yes, then that's another green sign

People with reputations and degrees will not burn their lives just to scam a few thousand dollars from an ICO. So if these team members have valid backgrounds, then you know they won't exit scam you.

Use GOogle too. See if other people are already calling the ICO a scam and find out why they're doing so.