If the you have CNTs in a liquid solution, it will form a suspension. The simplest way to distinguish a suspension and a solution is through Tyndall scattering. You can put the sample under ultrasonication for several minutes, and then use a light to reach the sample. It would be easier to see if you have a pure DI-water for comparison.
Place the suspension in a glass container. Take a laser pointer (red laser with wavelength 635 nm) and direct beam pointer through the suspension. All objects with size of about 60 nm will scatter light (Tyndall cone)