These are the giveaways:

1. They claim to have a secret way to make money.
2. They claim to have been successful in the past, but have no evidence.
3. They won't explain how their scheme actually works, other than using empty phrases like "arbitrage" or saying they buy and the price goes up.
4. Their explanation for why they want your money makes no sense. (Why wouldn't they just borrow money the conventional way?)