There isn't a sure shot way of knowing whether an ICO is legit or not. But you can definitely follow these things:
1) See if they have a nice and technically sound whitepaper. Whitepaper says a lot about an ICO.
2) Do a background check on the team. If they aren't visible on the internet, they have things to hide.
3) If an ICO has unrealistic expectations out of their token offerings, it means they haven't done their research properly. Shows a lack of dedication and planning.
4) See the advisory board. Big people means good ICO. No big shot advisor would want to be seen as a scammer.
For example, a new ICO is coming up. I stumbled upon them through some site. These guys are trying to make good use of blockchain. If you look at their site, it's clean and clear. Their whitepaper is very descriptive. Also, all the founders can be found on the net. It's a good initiative. Check 'em out here -
http://enkidu.io/