The weak scattering from smaller carbon nanotubes results in weak contrast in their electron microscope images, and observation and interpretation of such images require special attention in order to avoid erroneous conclusions. It is demonstrated that the 4 Angstrom carbon nanotube, residing inside a multi-walled carbon nanotube, bears recognizable signature in its image contrast for identification. For an isolated single-walled carbon nanotube, observation should be made in areas where the nanotube is exposed. When the carbon nanotube is overlapped with a supporting glassy carbon film, it is practically impossible for the nanotube to produce usable contrast features for unambiguous identification.