Proper crank arm spacing?
The gap you're talking about is more or less random and does not itself affect how symmetrical the Q-factor winds up being.

The quick visual check for Q symmetry is to eyeball against the chainstays, which is close enough in almost all cases to know whether there's a problem unless the frame either has an asymmetrical rear end or is smashed. To get an exact measurement, one uses the depth gauge part of a vernier against either the seat tube of a round-tubed bike or the back rim (after checking dish) for bikes with asymmetrical frames.