You can't even trust "winnings" in such cases: someone could create many trading accounts, and make random trades. If they lose, they abandon the account. If they win, they continue.
In the end they'll have many accounts with huge losses, and one or a few accounts with a large profit. That doesn't meant he profit is sustainable and will continue in the future. Even better: by the fact they want someone else's money to trade with, you can assume it didn't work out very well for their own money.