If you can see that the OP in the airdrop is requesting for a donation for verification purposes then that is a red flag for me. It is hard to really determine if an airdrop is a scam or not because some are just genuinely offering free coins to introduce it to the community. I know that with the airdrop announcements every now and then, it is not impossible that there is a scam airdrop just to get some information or donation from people but there are some who are legit because I myself join some airdrops too and receive the tokens so some of them upon being listed in an exchange, the developers just dump their coins while others are genuinely doing some development in their projects and the prices shoot up as well.