Not that hard, as most are scams and most are trying to do something someone else is doing better. But some more specific things:

1. A "discount" time period for ICO with limited coins, then price goes up
2. Airdrops, period
3. A whitepaper that overpromises
4. A roadmap that is unrealistic
5. A team with little experience/the wrong kind of experience


And always trust your gut-