Usually, a simple google search regarding the ICO, will help you find out if it's scam or not. Just google "[ICO NAME] scam" and there will be either no result or you will find out the ICO is actually scam.
Yeah I like this method as well
it gives you at least some pointers why someone think its a scam and if its discussion forums then you may get answers from someone that dont think it's a scam.