What are some of the visual cues I can look for to tell if plastics are aftermarket or factory?

SP bodywork on an R is an obvious or Rothmans on a 1996, but if the color scheme matches model/year and frame stamp....???

I need to be schooled

aftermarket fairings are thinner plastic a few of the ways they bolt together are different and the panels dont quite meet up perfect .
Paint prob is not the way to tell the oem fairings may have been changed with other oem or they may be painted oem.

http://www.dreamgate.ne.jp/NSR/general/NSR250a.html