For me, these are some things that I consider as red flags:

• Overpromising. The whitepaper is trying to illustrate something that I know isn't technically possible or rewarding to the market.
• Team members trying to hype the project by having the "most" people in the telegram.
• Most important, the token metrics allow seed or pre-sale to gain x10 or no lock. That's the worst because early investors for sure will dump on listing. E.g. Perlin. I expect listing day, their friends/families will dump at first light with their super gains.
• Inactive team members. Communication with public is almost non-existent. No idea what they're working on.
• Milestones unable to achieved and delayed for months. E.g. Look at Keep. They're supposed to launch in Feb but till now no news. That's more than three months of delay. Says a lot about the team capabilities.
• Projects who reserve a lot of tokens for themselves and sold less amount in the market. Without much circulation, if demand goes up, indeed the price skyrockets. But it only benefits the team and bear in mind it's a decentralised idea, you'll need more participants. If there's so little token circulation, how will project function well?
• Greedy projects who plan on raising a lot of funds for "just another similar idea". E.g. Bankera asked for $215,900,000 and thereafter raised 70% of it. Note: if you try their products, it's nowhere near fascinating. That's why such greedy projects are red flagged.
  

Hope it helps.