The issue is generally that the images are not sharp. If you use a telescope like Jay's links describe you get wonderful sharp images.
Most chemtrail believers are using quite low resolution digital video cameras only 1080p max with very low power optical zooms and they keep "zooming in" with the digital zoom to make the planes look bigger. However once the limit of optical zoom is reached there can be no more information gleaned from the footage (by amateurs) and the digital processing just "interpolates" the existing resolution images. ie it uses algorithms to smooth out the pixels once the limit of resolution is reached. That can work ok on a fuselage shape and large things like wings as the algorithm can safely assume a straight line of a wing edge of fuselage side will still be straight when interpolated closer but fine details are simply too small to pick up for most consumer cameras. The camera can't actually see them as they are "sub pixel" in size. ie, the detail as per the focussed image on the image sensor is SMALLER than the size of the individual sensor elements. The sensors simply do not have the resolving power to see them.
That's the same effect as how your eyes simply don't have the resolving power to make out individual trees on a distant mountain .
Go find a billboard advertisement on the side of a building that is similar in size as an aircraft marking and then try to video it from 10+km away. You will find a normal video camera is not up to the job very well. And that is probably a billboard that is flat facing you at 90 degrees.
Try something like a shiny cylindrical fuel tanker that is edge lit by the sun in the manner of an aircraft.
Aircraft are also highly reflective.. ie shiny.
Shiny objects reflect the surrounding colors. a chrome cylinder actually has no identifiable colour of its own. it is the colour of its surroundings.
http://4.bp.blogspot.com/_aQE-4419LRY/S_1m0zghE_I/AAAAAAAABM0/C772Ir7LaCs/s1600/Hirschgarten.jpg
A shiny object like a plane will have a lot of reflection of its environment
http://www.worldautocar.com/wp-content/uploads/2011/04/ferrari-white.jpg
In the case of aircraft that is blue or white hazy sky and that reflectivity will be an added effect to scattering and haze effects.
The planes are usually about 10km up and details like windows are flush with the sides of the fuselage, on the top half of a cylindrical reflective body and about the size of a human head or smaller. Even in footage of airliners landing it is sometimes difficult to see details like windows, let alone at an oblique angle.
A good example of a video that shows an aircraft that appears to have no markings and then obviously does have markings when seen much closer is
http://www.youtube.com/watch?v=o5O-DLk6cT8