Good question. I have several stages before making a decision.

Let's say, I browse an ICO calendar and go through projects one by one.

First impression
1. Website (design, style, template or custom)
2. What is the idea in a nutshell
3. Projects's social links (number of members, real or fake)
4. Team and advisors (if Vitalik is not on board:)

Deeper insight
1. Whitepaper
2. Where company is registered
3. Team's Linkedin profiles and past experience
4. Social activity, interviews and overall communication style
5. Token model
6. Advisors involvement into the project (are they just faces or their input is substantial)
7. Development plan, target market
8. Reviews on rating agencies if available