https://airdropalert.com/
https://www.ico-check.com/ico-airdrop
http://www.airdroptracker.co/
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.
If there is something that is suspicious then just ignore that airdrop and forget them forever. There are airdrops that are just paying those websites in able for them to be listed.
And these websites that are listing them must also filter those airdrops that they are listing.