My advice for you to tell which is the airdrop scam or the airdrop legit is to search for the details. Research for the information about this airdrop and know who is the one behind that airdrop. Be sure to secure your privacy when filling up the registration about the airdrop. If there is a easy task to join the airdrop without giving your private keys, i'm sure this is legit if also the creator is very trusting. If there are airdrop that want to get your personal information or any private information it can be a scam.