Dragon Ball Z: Budokai Tenkaichi 2

If you destroy some of the environment, sometimes a dragon ball will have been inside it and the radar will show you exactly where it is.

the radar will be shown in battle, at the bottom right of the screen. Randomly, when you destroy the environment, a dragonball will appear. It will be shown as a flashing yellow circle on this radar.

easy way to get dragonballs fast:
sayian saga, where you can go into the southern capital and fight General Tao. go into the stage with a strong character, destroy around, get the ball and kill Tao. rince and repeat as long as you want :-) Tao is a crappy fighter and wont put much of a challenge, leaving free to rao maround and search the dragonball. You can even go fight Tao as much as you want in a single scenario.