TYNDALL EFFECT
An easy way of determining whether a mixture is colloidal or not is through the use of the Tyndall Effect. When light is shined through a true solution, the light passes cleanly through the solution, however when light is passed through a colloidal solution, the substance in the dispersed phases scatters the light in all directions making it readily seen. An example of this is shining a flashlight into fog. The beam of light can be easily seen because the fog is a colloid.
Efecto Tyndall polo pó no aire.Wikimedia commons. CC BY-SA
Another method of determining whether a mixture is a colloid is by passing it through a semipermeable membrane. The larger dispersed particles in a colloid would be unable to pass through the membrane, while the surrounding liquid molecules can. Dialysis take advantage of the fact that colloids cannot diffuse through semipermeable membranes to filter them out of a medium.