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.
Airdrops are based on luck and possibilities, most of them looks so real, but end up disappointing people that participated. Very few of them project owners are out for the real deal with strong commitment to their project. Some are out to build their media handles only, others to scam unsuspecting participants.
You just need to be very careful and always DYOR and be sure you're following your instincts and not just what someone else says. Above all, don't cast your hope on it or participate with more resources than you can loose since some airdrop tasks are capital involving.
Airdrops pays, but you're not entirely sure the one that would pay well, so it's very much okay if you engage in more of them to increase your chances of making profits
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.
You can still do all these and it would still fail, but it surely helps, and the quality of information shared on their handles matters a lot also and can help you determine if they're building or just marketing a baseless project.
Their partnerships are also key to the success. If they're able to secure good partnerships with well meaning crypto enthusiastic companies, it surely shows they're going somewhere good.