I use BlackRapid Camera straps, and just noticed that the company announced the launch of LensBling, a series of rear lens caps that allow users to quickly and easily differentiate between lenses they own. The rear lens caps carry bold graphics and labeling so it's very easy to identify and quickly find the right lens in a camera bag.
Alternatively, one can do what I do...and that is to print labels with the focal length of the lens, and stick these to the lens' front and rear caps (and sometimes the hood as well).
Notwithstanding, the LensBling is much more elegant and only cost $8.50 each. Not bad.