I didn't know there are sites that provide airdrops updates
You can google everything that you want to know including those airdrops. I did the job for you and here are the few websites that came out.https://airdropalert.com/
https://www.ico-check.com/ico-airdrop
http://www.airdroptracker.co/
i hope they are legit 
What you should doubt are those airdrops if they are legit. There are cases that they are asking for a payment before you get those airdrop tokens.
Yes, but whether they are listing the good Airdrops because most of the coins after distribution don't have any value. Mostly there are many fake coins listing in those Airdrop website and we need to find the good potential coins in order to be worthable while selling them.