Whitepaper, technology of the ICO, and team.

If their whitepaper is overly fluffed with no real content talking about their product, it's a red flag

If they do not have a detailed example of what their technology will be doing -- and I'm hoping it actually NEEDS to use the blockchain in some way -- best to avoid it.

A solid team, like a good idea, brings on a good foundation for the company to grow. Check out the team members and make sure they have experience in the industry this ICO is focused on.