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.
of course with the number of ICOs in the crypto world there may be one ICO scam and that there must be with the possibility that ICO is not too visible to investors to invest with the decline of ICO for the coming day is certainly very disappointing.