How can you can tell the difference between a solution and a clear colloid?
Subject them to testing by dynamic light scattering.
DLS should give no output for a solution (or at least very little output - there may be some scattering resulting from impurities in the solution, maybe the odd bit of unintentionally added dust or something like that), but will give a definite result for a colloid which contains stably dispersed particles.
On a light intensity/time curve, the colloid should give regular peaks, whilst the solution should be essentially flat (give or take from impurities).
DYNAMIC LIGHT SCATTERING (DLS)
Dispersed particles in a colloid exhibit Brownian motion. If light is incident on the particles it become scattered by them, and the motion results in the scattered light from different particles being in or out of phase, cancelling out or reinforcing, so that a time dependent pattern emerges.
Dynamic Light Scattering measures these time dependent fluctuations by fast photon counter. The fluctuations relate to the rate of diffusion of the particles through the dispersion medium, which is in turn related to the hydrodynamic radius of the particles from which mean particle size can be determined.
A pure solution will not scatter light in this way (except for some artifact scattering caused by impurities such as dust).