Most live casino players are legit and do not fake their big wins. However, there are fraudulent streamers, too, which cast suspicion on the legitimacy of even the legit streamers. Although it's a bit hard to know a fake streamer, some signs can help you identify one.
Suppose a streamer only plays on a low-quality casino of which some do not comply with the law and responsible gambling policies and win a considerable amount of money. In that case, that is most likely fake money.