How can you tell just by looking whether a mixture in water is a solution or a suspension?

If the mixture is completely homogeneous, then the mixture is true solution as in the

solution the solute particles are completely dissolved and have uniform composition.

These are transparent.

A colloid is a type of mixture in which one substance is dispersed evenly throughout another. These are mostly translucent, for example:-milk.

A suspension is most easy to identify since the solute particles do not dissolve in the solution. They either keep floating in the solution or settle down at the bottom.