And sometimes it can not be decided whether a project is a scam or not, since some might just fail because the team is bad, but that doesn't make them a scam.
Of course you need to judge the implementation of the final product. Many projects are under development for a long time ... sometimes for several years. And sometimes it is very difficult to understand whether these projects are deceiving their users or are really actively developing.
And that is what you just are not able to do if the product is under development for several years. You have to wait till the end then and if no proper updates are given you will lose trust and confidence and sell.