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.
There is no a hundred percent guaranteed to distinguish if the ico is scam or not but better if you do some research about the ico that you will trying to join to avoid scam.