It seems like a fine line to me - how can you tell whether you're overstocking or understocking? Space wise, filtration wise... I don't know if I'm missing something but everyone on forums always seems to just know if they're stocked correctly. I never do! If someone lists off their stocking list, I can't ever tell if they have too much or not until someone else says and confirms it for me.

Under stocked ( No Problem )
Over stocked ( Potential Problem )
It all comes down to common sense; Fish to big for tank / Too many fish in tank to manage acceptable water parameters.
The solution is education. Learn desired species growth potential, Space needs, Compatibility issues, Aquascape requirements, and most importantly exercise restraint at the fish store. Resist impulse buying.
Essentially, an over stocked tank is one whose occupants produce too much waste for the fish keeper to manage.