To be able to distinguish which airdrop is a scam or legit before you start referring other people, the first thing you have to pay attention to is that if you have to deposit crypto then it means you are buying and most likely the airdrop is a scam. Next, you can review their official website, evaluate their various social media and make sure their followers are not bots, visit their whitepaper and check their roadmap to see if their project is running according to schedule/plan. Where they sell tokens whether on their own website or a well-known exchange. If it's on your own website, it's most likely a scam. You should also use an empty wallet to avoid dust attacks.