On the menu screen hold the joystick in port 0 to the left and the joystick in port 1 to the right, for about five seconds. A cheat menu will appear, enabling you to set player 1 and 2 to be the evil maximum, to toggle the CPU as player 2 or to alter your credits and energy.

If the characters have shadows beneath them whilst playing, then you've got a copy of the enhanced version. Hold the joystick in port 1 down, until the cheat options appear.