Well, actually there are no specific ways to determine fake giveaways or fake airdrops. You can do your own ways or follow other suggestions. But the point, how you can find the signs or proofs whether it is fake or real ones. However, if you want to avoid the fake ones, you mustn't join giveaway or airdrop that you don't really know the projects. While if the giveaway or airdrop was announced officially by a trusted crypto project, ex: a giveaway from a top exchange, then it is no problem to join. As long as you find the information from the official site or social media channels of the project.