Are there any differences between real and fake smash balls?

In a real Smash Ball, the vertical stripe of the "brethrens' cross" is thicker than the horizontal one, and it's further away from the center of the symbol. In a Fake Smash Ball, the thickness of the stripes is reversed.

This can be harder to distinguish if the ball is rolling around on the ground, but even then it can become clear that the fake cross is the mirror image of the real.

In addition to the visual differences that Toomai mentions, the fake smash ball will also have the tendency to float towards players and action. It'll even put on a burst of speed at times to try to get hit by your attacks!

This will be a little more subtle when you're in the heat of a fight, but every identifying factor helps! See the Smash Wiki for more trivia.