How can you can tell the difference between a solution and a clear colloid?

DLS should produce very little or no output for a solution (there may be some scattering from impurities in the solution, such as the occasional accidentally added dust particle or something similar), but it will produce a definitive result for a colloid containing particles that are steadily dispersed.

The solution should be nearly flat (give or take from impurities), while the colloid should show regular peaks on a light intensity/time curve.

DYNAMIC LIGHT SCATTERING (DLS): Dispersed particles in a colloid move in a Brownian manner when light strikes them. This motion causes the light to scatter, which causes the scattered light from various particles to either be in phase or out of phase, canceling out or reinforcing, resulting in the formation of a time-dependent pattern.

By using a fast photon counter, Dynamic Light Scattering measures these time-dependent fluctuations. These fluctuations are related to the rate at which the particles diffuse through the dispersion medium, which is related to the hydrodynamic radius of the particles, which can be used to calculate the mean particle size.

With the exception of some artifact scattering brought on by contaminants like dust, a pure solution will not scatter light in this manner.

The main difference lies in particle size. Solutions have particles smaller than 1 nanometer, making them transparent, while colloids have particles between 1 and 1000 nanometers, causing them to scatter light, appearing translucent or opaque.