Its a bit subjective, a lot of the opinions people will give also relates to how much risk they are willing to take. Red flags are common sense really. You should really know the ins- and outs of if you want to make responsible choices which also boils down to experience and knowledge of investing. There are so many red flags it would be hard to list them all so just use common sense and always ask why, if some whitepaper looks like a brochure with fancy marketing language then ignore that project and be aware of any ICOs that are flashing with industry names (like Roger Ver) these companies only do that to get your attention and your money and often those situations don't work out well. The example I give you now also illustrates how subjective it is and that it boils down to experience. Try to become familiar with the space first, maybe look up past scams and analyze them for their red flags so you can learn from the mistakes of others.