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.
I guess the best way to avoid ICO scam is not to invest in ICOs at all 
That's the only way to be sure. And it is not really as bad as it sounds. One may still explore the ICOs which seem interesting and invest in them when the coins are released. I think that mostly the prices don't differ much from what they used to be during ico when the coins start to appear on exchanges, whereas the safety obviously grows a lot since you can be sure you'll be able to sell the coins just as you were able to buy them.We can't even earn huge if we just really on single investment and ICO's are much profitable if we select those coins who are good and have a proper roadmap for there project. But we can still earn by participating on there bounty campaign and this is much more profitable rather than investing some money on some particular ICO's