Roughly speaking for now there is no way to know the altcoin or token you got/buy from the project is a scam, except: You check all the origins of the project in detail before you make a decision to join or buy tokens to become an investor, it's a long process.

Suggestion:
If you want to get altcoins/tokens from projects/bounties, at least you can see the manager who manages the bounty who has a good reputation or not in this forum, at least it can have a good chance of tokens/Altcoins in the market and also the team involved in the project.

With advice like that at least, you are a bit spared from Altcoins / tokens that lead to scams, because managers who have a good reputation will assess the Altcoins/tokens/teams involved in the project carefully, before they make a decision to promote the project in this forum.

Note:
What I tell you does not guarantee 100% that you will avoid scams, sometimes managers can also be tricked by the company after they successfully promote their project, but good reputation managers at least they do Escrow, for guarantee.