The main point is not how to identify good airdrop but how to find good project. Then you can check whether that project has an airdrop/bounty campaign or not. And to find good project is not an easy task especially nowadays...You need to carefully examine project whitepaper, its dev team and their background, try to assess the possible project contribution to the crypto world and community and so on. Only after that you will be able to make the right choice and can count on good profit in the future.