If the project has a clear direction of where it's heading at and plus it's got some purpose then usually it is legit.

But these sort of airdrops are rare and usually will require you to hold an amount of some other existing altcoin or bitcoin. That's not an airdrop since you need your own money but they do still call it a snapshot, or an airdrop.

Just be warned though, most airdrops are indeed going to be ones that are trying to get your email or whatnot. Not going to be worth it to invest your time in those. Better off actually investing money into better projects.