To be honest, it's hard to say if an airdrop is legit or will it make it big or not.
But we can surely identify the obvious scams as airdrops are meant to distribute coins/tokens for free.
If any airdrop is asking for any payment then it's probably a scam.
Projects which have a good experienced and dedicated team with an unique idea are likely to make it big.
So if you find any such projects giving out airdrops then you get into it.
While it's easy to see the obvious scam ones, it's very hard to find legit ones among all the ones that look legit but scams. I know that it is going to be very tough to handle all of this because we are going to have trouble finding something decent. Even the legit ones could fail too, so success is not something that is going to happen that easily, we are going to end up with a result that will not be all that simple and we could end up with some trouble on the long run as well.
We need to figure out a way to see this getting any type of good result, and we need to make this work somehow as well. I believe that we are going to see this changing eventually, and it is going to end up being a little bit of a trouble in the end as well.