You always have to check if they have a legit website. If they have a good team behind it. check their whitepaper and roadmap. Most importantly they don’t ask for donations.

Do you have a coin in mind? DeepOnion does legit airdrops for completing simple tasks and having 50 ONIONs in your wallet. I know some other coins do airdrops and I know there are probably some scams out there. You need to research then ask if anyone has experience with that coin.