You can not be 100% sure about the real ones and sometimes, it's the one that you don't expect to make profit from that is going to give you the profit you don't expect. 

To know  a real airdrop (not 100% tho), you need to follow up the project's update. Do not just keep participating in the airdrop when the project you are promoting is not even showing a sign of growth and they are not developing their project. Follow every information on their social media accounts and also ask questions in their most community.