Most experienced airdrop hunters can easily spot a scam or a fishy airdrop campaign. Sadly, a lot of newbies easily fall for these fraudulent campaigns. OP already mentioned some good tips to avoid getting scammed, I just want to add some.
register or participate in airdrops only in well known sites
Make sure to double check if it is legit. Better bookmark the website of these legit airdrop platforms.
read through the form before filling it. If anything does not look right to you, just leave that site immediately
Avoid any airdrop that asks for your private key or ask you to log in to your personal wallet.
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If you can, try to report also suspicious airdrops you receive in your emails like "Etherdelta airdrop; Rebellious airdrop; EOS airdrop" so they could be taken down as soon as possible.