You will never be 100% sure if an ico is a scam or not. Of course there are indicators, some ico's are just so obvious that it's funny but others look extremely promising but then they suddenly fall of the grid. I don't really mess with ico's, I happily hold the majority of my assets in bitcoin.
Yes, you can scan it through reading its whitepaper and try to understand its content, once you notice that it has so much promising content that is not that easy to believe, consisting an offer which is above your expectation, there might be a possibility that it is a scam. You should be careful on choosing the right ICO.