Sometimes even the team working on something doesn't realize its a scam. Like the idea of the project may simply just not work.. or the team themselves may be working on something but the lead dev may embezzle or misuse funds. I would say the warning signs aren't always clear but some are clearly a scam. Avoid anything that guarantees profits.. or guarantees results.. there are no guaranteed returns in life. Also be active in an ICO's community such as here, Reddit, and Facebook. People tend to investigate things pretty far if they're looking to invest and they will be VERY vocal with all of their findings.... you just need your pessimism glasses on and the ability to find it.