1
Anything that asks you to pay a fee to release the prize
The single tell that ends the conversation.
Processing fee. Delivery charge. Customs. Insurance. Administration. Currency conversion. There are a hundred names and they are all the same request: send money to receive money.
A real sweepstakes cannot do this, and not because it would be rude. Requiring payment to collect a prize is consideration, and a promotion with prize, chance and consideration is an illegal lottery. The fee would convert a lawful promotion into a crime.
So this is not a judgement call or a matter of degree. If there is a fee to collect, it is not a legitimate sweepstakes, regardless of how plausible everything else looks.