A second way is for one player to make the sign behind his hand (fig 2) and the other player must guess which sign it is.A third way is for the two players to face away from each other. A third player must announce the winner based on their signs (fig 3).
The Greeks also played a similar game. Here they appear to have had a method of keeping score. Each player held one end of a stick which was probably numbered. If you won a game, you advanced your fingers on the stick one number. If you lost, you moved your fingers back one number. The lady on the right in figure 4 is winning, she has more of the stick than her partner. You can make a stick by making marks on a piece of dowel rod every 1 or 2 inches, or, if you just want to try the game out, use a yard stick--it is already numbered!
†Source: Falkener, Edward Games Ancient and Oriental and How to Play Them. Dover, 1961 (originally published in 1892).