As you know most of airdrops are scams and not paying after complete there ICO's, so when you have to join airdrop you should find the all links are working and ICO ranking websites .
We can't trust airdrops unless it do have legit teams handling over a legit sites which also includes being active at the social media. The best options to do is always trial and error registration in order to determine if they will give us reward airdrop coins after certain registration. Buf if some forms asking few amount of eth, never transact with them and consider them as scammers already.