By checking what value that airdrop brings to the coin or token doing the airdrop. A good airdrop will bring more value to it then it is losing due to giving away a lot of coins.
A good airdrop won't give away free money without having anything to benefit from it. Good airdrops will have the person receiving the airdrop do something for the project in return for receiving the airdrop. For example, register for a mailing list, sign up to the platform etc.
Good airdrops will prioritize giving the airdrop to people who have worked on projects similar to it or are potential investors. Good airdrops might also require the person to have a small balance of another cryptocurrency to prevent sybil attacks.