For me, it's the team's background and the companies backing them. I know a lot of projects who claimed to have an impressive team, but none of those people were real. I take each and every member, inspect their LinkedIns and see if everything over there is true.
After I'm done with this, I check the backing companies list. I go to their official pages and see if they made any announcements about a partnership. For example, there was this project, Ludos Protocol - if I can remember well - claiming that they were backed by Softbank. I went through PR releases and directly to Softbank-related sources, as it was too good to be true, and found out that they were serious.
The general idea is to do your research and when something that seems too good to be true occurs, insists on that. There are tools you can use for researching, like Buzzsumo, or even an advanced search on Google can do the trick.