At a point the team start given excuses to abide by what they promised to do or stop communicating with the community then I can sense the project is scam. How do you determine a project is gone bad and won't fulfil it promise? What criteria do you use ? or you wait for years thinking all will be well ?
For scam project there is no hope that you could say yourself all will be well. they just got your money which is not gonna back to you. There is not special thing that you understand very soon because they are just already planed to cheat you. but yeas first they seems like very helpful but after they stop commiunicating to you, never replay to your question, not gonna active in social media, inactive in telegram, change survival time their ICO/IEO/STO date, fake promisses to you, not proffesional for any kinda job and so on.
Normal thing for you to experienced on the earliest time as possible where you do get active responses from the team but after the sale then they dont care about you anymore.Realizing if the project is scam or not at first is really a tough job yet you wont able to know if they do have that motive on the first place or not.
We cant detect 100% scam projects when they are starting yet no matter how you research you would still have the chance on getting victimized.