Payout percentages are calculated as a 'return to player rates'. In order to find out the payout percentage for the game you have to compare the winning paid to wagered amounts.
The comparison can be made for all games at the casino for a certain period, particular game or a group of games.
For online casinos powered by Microgaming they review the payouts in four groups:
The main idea of publishing these reviews is building the customers' confidence that results of the games are produced by Random Number Generation (RNG).
Of course, these numbers reflect only past results for all (or group of games) for the certain month and do not reveal the outcome of any individual game or future payouts.