One of the easiest way to prevent being scammed (other than avoiding fishy promotion at all cost) is by doing a quick search on google or this forum about the site you're about to invest. Suppose that you're about to invest on a site called xyz, a little research on this site will mostly tell you how legit the site is. If it is scam, there will be lots of post telling you that xyz is scam. One quick reminder, though, is to never trust invest site which offer irrational ROI.