Because teams take it in turns to play an innings and potentially score runs, there’s no purely objective way to tell who is winning in cricket matches at a given point. However, several statistical measures can give a good idea.
During the initial innings of any game, spectactors can’t compare the batting team’s run scoring to that of their opponents (because it hasn’t happened yet!) However, they can get a good idea of the batting team’s performance in isolation by comparing it to a “par” score. This isn’t a purely objective or official figure, but rather something that cricket analysts and fans will discuss among themselves. In principle, the par score is what an average team with an average performance would be expected to score given the weather and the state of the pitch and outfield.
You can compare the current score and the number of overs to the par score at any stage of the innings, though it isn’t simple a case of extrapolating the current score and overs played to a potential total innings score. That’s because, depending on how many wickets the team has left, it will usually play more aggressively and take more risks towards the end of the innings when the damage of losing a wicket is comparatively less important. A common rule of thumb is that a team playing 50 overs would be expected to wind up with double the score it reaches after 30 overs.
In the second innings, it’s much easier for spectators tell who is “winning” without having to make assumptions or do calculations. That’s because the scoreboard (at most high level games) is constantly updated with a “DLS” figure. This represents the score the batting team would have to beat if the game was abandoned at the end of the current over and the Duckworth Lewis Stern (DLS) system was used to decide the winner.
The figure will change depending on how many wickets the batting team loses. As a rough but reliable indicator, if the batting team is ahead of the “DLS” score at the end of the over, it stands a better chance of winning than losing. The more it is ahead, the more likely the chance of winning (and vice versa.)
In a multi-day game such as a Test, there’s no DLS and “who is winning” becomes more complicated because of the possibility of a draw. Some TV companies use a technology such as “WinViz” which forecasts the percentage chances of either side winning (or a draw) at any given moment. This can be based purely on the runs scored so far, the wickets lost and the time left and how games have historically ended from similar positions (such as the Winning and Score Predictor (WASP) tool). Alternatively some systems also take into account historical data about the specific two teams playing, the venue (which can affect the likely speed of scoring runs or losing wickets) and even the weather forecast. Such systems can be helpful as a shortcut though some viewers argue the percentages swing back and forth too quickly to be reliable.