how can we tell if the project is a scam without investing in it?
The classic team info, WP plagiarizing etc is one of them. While it might be more complicated nowadays, giving them money and waiting to get scammed is undoubtedly not a good idea. You can also simply wait and see for others to test the water if you're too lazy to check whether a project or service is going to scam you or not (maybe not the most ethical way to do it though).
Scammers can sit their asses or hire a good writer and get an original white paper and still scam with it; there is no limit to what these scammers can do. There were some highly rated projects that went on to become shit after a successful ICO, and there were projects that received no ratings and went on to become great. However, as you stated, allowing people to enter first while you observe is the best option.