There is no way to ensure that you pick a winning slot. If there were ways to tell which games are going to payout soon, casinos would be bankrupt. That said, there are two factors you can look at to statistically improve your chances of picking a winning machine – return-to-player (RTP) percentage and volatility.
Online slot games with a high RTP percentage and low volatility have the best chance of paying players out. Usually, these games don’t have the largest jackpot prizes, though.