Over the century, spacecraft has evolved from rough assemblies of electronic and mechanical parts to completely integrated smooth-looking systems. As technology has been improving, spacecraft have been built with more taste, and an experienced eye is able to tell the age of a vessel just by its design.
The older spacecraft, built before the 2060s have a very functional look, with bulky girders and apparent pipes and radiators running along the hull. Early engineers cared more about making the vessel fly. Detectors and communicator antennae were still typical parabolic mirrors, though they had already begun to be replaced with flat phased-array systems.
In the 2070s (for example the Sudbury-class craft), most systems are already integrated, and structures disappear. Although, appearance was not an issue in this time. Hull radiators were still visible, and though the first droplet radiators had appeared on military vehicles, wing radiators were still often large fins. The first hemispherical phased-arrays appear in the mid decade, replacing flat plates.
The 2080s (see for instance the SDV-90) are a transitional era: maturation of fabrication techniques allowed hulls to be more seamless, and aesthetic began to be taken into account, even for military spacecraft, for which an aggressive look was the rule. As shape is not much of a problem in vacuum, marketing experts demanded that hulls be more attractive to the potential buyers. Girders were not assemblies of composite elements anymore, but were directly grown into the wanted shapes. Efficient heat-transportation technology allowed hull radiators to merge in the hull and become nearly invisible, and fins were eventually all replaced with aerial-looking liquid-droplet radiators.
It is in the 2090s that spacecraft began to really look streamlined. It became the standard to make pleasant, fast and/or aggressive looking vessels with little apparent structures. This style is best represented by the LSDV-5.
This evolution of design has been followed by American and European spacecraft companies. The Chinese have shown less interest in the look of their vessels, which today still have functional-looking appearances. However, the difference between periods is evident due to the general integration of the systems, the organic-looking girders and the liquid-droplet radiators.
There are other ways to tell the age of a spacecraft. After many years of vacuum and cosmic ray exposure, plastics decay and liquid crystals crack and break; and some pieces and plates have to be replaced by models looking different, or just removed, leaving apparent scars. Metal tends to stick to metal after long vacuum exposure, and hatches that have not been opened for a long time could be stuck forever. Hulls are spotted with micro-meteoroid impacts and stains from cooling-system leaks, and military vessels show burns from laser shots...
Nemtos