After quite some time in crypto, we can say we have seen almost everything on how crypto-scammers are stealing money from crypto-enthousiasts. Here are the easy ways to find out if a site is a scam, even without joining:

Airdrop Sites

Airdrop sites will ask their users to perform tasks in exchange for free crypto-tokens. As a user, you need to see what those tasks are. It may be simply taking the time to watch ads, and get paid just a few satoshis (and the more intrusive and malicious the ads the higher the 'reward'), or signing contracts that secretly mine coins by using your browser without you knowing. Usually, those sites do pay something, but this something is simply next to nothing (all your efforts may worth 1-2 cents for hours of watching - even the most malicious ads - unless you bring hundreds of even thousands of users as a referral).

Exchanges

Just like any other exchange not offering 'exotic' rewards, they will ask you not only to join, but also to bring them your friends or contacts to join. Don;t be fooled by the rewards. The more exotic a reward offered is, the harder or more expensive to get it. Other exchanges call their programmes to be 'reward' or 'referral' ones, but in fact these are limited-time revenue sharing programmes, where you as a referrer have no idea about the revenue generated by those who you referred. It's basically, 'whatever they tell you'.

Coin and Token Sites

Usually, they are connected to exchanges, but sometimes you may find them as standalone offers. Your task here is to invite again your contacts to join the coin/token supporters, as a way to get an audience for their upcoming ICO. Although they will deliver the promised coins or tokens, the contract they sit on is locked. In practice, you are forced to keep those coins or tokens for as long as they need until they whitelist transfers to a partner exchange, where they will be taking a cut of the transaction fees or bid/ask price difference.