I'm not sure if this will make you easy to decide which legit airdrop but the majority of airdrop look crap in this time. There will be no more airdrop will become a legit project in the future just like decred. As you can see the majority of airdrop right now doesn't provide the business plan that makes sense. They are having only interest to raised a lot of money.
But look at the whitepaper and business plan.