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.
With knowing that everything has risk, we can make actions to lower the risk. When we
speak about ICO`s research is very important. Superficial or deep, how much effort you invest
in research that much you can lower that risk`s.
Until now I have 5 successful bounties behind me, I didn`t make mistake when I was choosing
bounty campaign. I invested my money to, that`s how much I was sure in my decision.