Checking for legitimacy is an objective thing, but there are some guidelines you can use. Check out their website, development team, project roadmap (whitepaper), social media pages (followers, activity, etc). Also tread (more) carefully whenever an airdrop asks for a lot of identifying information. I believe the difference is traditional airdrop requires you to fill out a large form often times asking a lot of identifiable information (which people don't like and many times turn out to be scams), while self claim airdrops require you to just send a small amount of ETH through a smart contract and in return tokens are transferred to your wallet.
Check out our blog post on airdrops to learn more:
http://bit.ly/AirdroppingAlso look into this bitcointalk thread about airdrop scams to help you know what to avoid in the future:
https://bitcointalk.org/index.php?topic=4437085.0Cheers
