One should be able to make the difference between scam and legit projects because they're obvious unless a scammer is actually putting so much effort which is usually not the case. An airdrop requirement are usually signing messages, filling a form etc. It's nothing hard. As for signature campaigns and other bounties, there is no reason on why they won't give you your coins, as they could create them out of thin air. The only scenario where you could actually get scammed is while investing in an ICO, in that case, you should do a background check on the escrow, team and read the whitepaper which most people don't do.
so that case isn't called as scam project instead of what? and what is legit project?
