newbie aperture question

Hi,

I'm currently using a hand me down 6" reflector (f3.5?) and when
conditions are good I can see faint banding on Jupiter. This is using
an 18mm eyepiece. (30x?) I live in an apt complex in San Diego county
with plenty of light pollution from street lamps, strip malls, etc.

My understanding is that a larger aperture telescope, say 8", would
produce a clearer picture of jupiter at the same magnification
(assuming I switch eyepieces).

But it seems as if there would be a point at which atmospheric
conditions and light pollution are such that having a higher aperture
telescope would not produce any clearer an image at a given
magnification.

So my question is if there is any way to tell whether image fuzziness
is a result of the scope or of the environment without looking through
a larger scope. In other words, how do I know if my scope's aperture
is the limiting factor?

Thanks,
Eddie.