what is the best method to know SCAM [AIRDROPs] 
PROJECT
almost all of them is scam and just want to take money from investors thats why they created a token . i suggest not to invest in any of them if you are investors try to find a legit project. PROJECTYou say "they want to take money from investors" but airdrops are free as far as I know. Something what comes for free can't really be scam.
ICO can be scam but that is something else.