There are multiple factors that plays in the background. Let me list a few for you.
1. Team: It is the most important check you need to conduct before you invest anything in a particular ICO. Check the linked in profiles of the team members. Fake and newly created accounts can be easily identified. So if you see all profiles were new, most probably they are fake and created for this ICO only. That's one red flag.
2. Real world business: There are many ICOs running currently without having any real world business. They are promising to create a platform for something related to technology and have a roadmap like any other ICO. Follow their twitter account and see their activities. I have seen few ICOs like BananaCOin, Bankex etc, are really active in the social platform and continuously updating their activities. They post the pictures of their offices, press meets, Business meets and various other activities. This will give you an idea about the company, whether it's real or someone is operating from their home to scam away some money.
3. Product: This factor actually depends on you. I myself is confident in investing in exchange platforms. These type of business may have some future feasibility. But if someone is promising to build an AI for the blockchain and use a lot of technical verbiage to specify their products without showing the business side of it, keep them in your avoidance list.
4. Intuition: This factor belongs to you only. Use your own intuition before investing in any ICO. Hear what your gut feelings say and proceed accordingly.
Hope this helps!
That's a nice list. I understand that you said that you'll just list a few but I feel like it is missing an important factor. And that is their Roadmap. You need to look at the roadmap of an ICO because this is where they place all the important dates for their project. Like when are they going to release their product or when are they going to develop their product. These are important because you will know if the team behind it really has plans for the future. If they really plan on improving their project.