The best indicator that I use to determine if a campaign is scam is if the team has not shown themselves in public. I'm not talking about the LinkedIn profiles, but rather event pictures with the team and interviews done.
I'm always looking for videos or pictures that showcase the the team in events or even interviews.
There are some ICOs that have anonymous teams, and I don't join them since they have higher chances of scamming.
It may be a bit late to join a campaign after seeing the team in action but at least the campaigns I have joined in using this method have been successful.
This is what I came here to type and I'm glad I decided to check the existing replies before penning mine. I have been involved in a scam campaign and the first thing I realized after it was discovered was that the team never put themselves out there. They didn't attend any events or conference or anything. I must say that they sold a very big dream and I guess many people (including myself) were to excited to notice the other flaws.