Research. No way to be 100% certain, but read whitepapers, talk to devs, etc. The less info you can find, the greater likelihood of a scam.
It is true that scammers runs a work all over the world, but the good thing to do is to researh becasue it proves the identity 100%. Well ICO's is known if it is legit. If you joined into a bounty campaign you will be paid by ICO in which you can converted into Fiat money or hol until it will grow higer its up to you. Their are different strategies to know if it is true to all or scam, just read an updates to know the real status behind it.