Transparency
Real community
Have working product
Could able to spend to market or advertise their project.
Fake
Lack of transparency
Created the same name of animals or other famous token who became hype
Questionable reputation of the owners
Ask important details or funds to their participants
Everything seems unclear on their project
About that real community: Especially larger projects have fake copycat communities in telegram and discord that have at least as many members and have sticked topics about airdrops that are fake. Always find the community via official links like trough coingecko or coinmarketcap.
There doesn't need to be a working product though. Some projects take years to build, yet they have airdrops. They might not be profitable airdrops as they don't want to flood the markets with free tokens and when it's a public, pre-announced airdrop, number of airdrop participants render the whole effort worthless for participants.
Advertising volume isn't really everything, but smart advertising for right people is. Depending what they are planning to build, it might not make sense to make huge advertisement campaigns.
Anonymous or pseudonymous team is usually bad in long term. They might try to explain it by saying things like "satoshi was a pseudonym too", but from 999 out of 1000 it's a bad sign.
Coverage by at least one good and established crypto news site or respected influencer. If no one has heard about this public airdrop, there's are good reasons for that, and none of them are good.
You aren't most likely losing anything by skipping something you aren't sure of. So you might as well consider most of them fake. Best airdrops out there are not pre-announced, but they are just rewarding the supporters from the start.