How can you tell whether a fat contains primarily saturated or unsaturated fatty acids?

In terms of physically observing it, if the substance is a lions based substance, like oil or margarine, depending on the state, this will inform how well the hydrocarbons can pack together well or not, or will there be bends in the ring preventing the molecules from packing well.

Liquids are unsaturated
Solids are saturated.

Vander waals forces are inter molecular bonds that exist within molecules of the same substance. a water molecule is made up of oxygen and hydrogen atoms which share electrons between each other. the vander waals forces are forces that exist between multitude of different  molecules of water. dipole dipole forces is the type of vander waals force that exist in water. the oxygen atom which is more electronegative pulls the shared electron closer to itself thus generating a partial negative charge while the hydrogen acquire a partial positive charge. these two different charges causes attraction to occur in water molecules.