I think that a trustworthy ICO should have a nice website with some introductory video, whitepaper, roadmap, normal team (experienced and/or educated properly), and an original idea. The token distribution should not let the team and investing companies take the control package of coins, so that the price is not manipulated. It would also really help if ICO had a rich famous partner.
For me it is easy to trust if you can see a good result as well as a good feedback.
Well, the thing is that feedback usually appears after the ico stage, when the project does and doesn't make it to the market. What we can see before is only some general attitude of people to the ico, their expectations and beliefs. Now, that is not a very trustworthy thing.
Before I could trust an ICO, there must be a regulatory authority that governs the affairs of all ICO, which means every ICO must have registered and pass through the thoroughly screening from the authority, by then I'll trust ICO.
That is pretty fair, but even if the government checks the projects for you and declines potential scam, the system cannot be working 100% and even if you invest in an ICO which is not going to scam people it still doesn't mean that you will not lose your money.