How can someone get to know when an airdrop is available?
https://airdropalert.com/About the date of when airdrop will be available, and it can't be predicted that depends on the developer itself. Because of all decisions will be on the developer itself about the distribution, registration and others.
Which telegram channels? Is there any specific?
Not sure about that but it seems the majority of telegram channel which gives you airdrop list, and the majority of them are shitcoin. It may better to keep your eyes on the airdropalert site.