A better question is how do you know your algorithm is successful? If it predicts correctly 10 days in a row, have you got a winner? It's like those guys that advertise by saying "We outperformed the market by 6.2% on average the last four years." Put enough monkeys in front of a trading terminal and you're going to have some that outperform the market by 6.2%; doesn't mean you should invest your money with them.

Have you ever wondered how every fund manager has a fund in the top 25%? It's easy, they launch several funds in one sector, then after a few years merge them all into the top one, and use its historical performance for the marketing materials not the average of them all.

You can't. You can only know your algorithm was successful. The market is changing, so even if you could prove, given variance and sample size, that your algorithm was +EV and not merely lucky, you would only be proving that it was +Ev and not merely lucky. Not that it will be in the future.

That's why you look at sharpe ratio, rather than absolute return.