As stated in the topic, what is the first thing that makes you raise the alarm?
Actually it's hard to identify if the project is scam. They can pretend to have a very strong, responsive and friendly support. Has a transparency, white papers and road maps are realistic. But there is still only one thing left in this type of projects, it should be backed up by anyone who has a positive and big trust rating in the market that will serve as guarantor and escrow.