As far as i know, i don't have experienced yet to participate in a campaign that ends to a scam. But to answer your question, it's really hard for anyone to know if the campaign is legit or just a scam, but if you make a lot of research about the project that you want to invest in, like reviewing of their whitepaper and identifying those members behind the team if they are credible and not fabricated only, this will reduce the risk that you can take from investing in this project.