First is the idea of an ICO, imho. If it's something new and revolutionary than it can be easier to understand is scam or not. I doubt that revolution ideas will be used in scam ICO in total with other parametrs you should check.
You should check it's Whitepaper and Roadmap, evaluate the activity of developers and promotion in different social media. Search info about developers (maybe it's on linkedin). Invest only the money you are ready to loose.
Also for the those who are willing to go in ICO with big money - you need to order Audit from special companies and check is it worth or not. Without real audit - every ICO is not more than an ordinary lottery.