How can you identify cis and trans isomers?

Both of the isomers have exactly the same atoms joined up in exactly the same order .It means that the Van der Waals dispersion forces between the molecules will be identical in both cases.

The difference between the two is that the
1) #Cis# #Isomer# is #polar# whereas the #Trans# #Isomer# is #non-polar#.
2)Cis isomer due to being polar shows Dipole moment while the trans isomer does not.

Example:

Consider the case of 1,2-dichloroethene

In one, the two chlorine atoms are locked on opposite sides of the double bond. This is known as the trans isomer.

In the other, the two chlorine atoms are locked on the same side of the double bond. This is know as the cis isomer.

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The effect of geometric/cis-trans isomerism on physical properties

cis 1,2-dichloroethene has a melting point of #-80^@C# and boiling point of #60^@C# while trans 1,2-dichloroethene has a melting point of #-50^@C# and a boiling point of #48^@C#.
In each case, the higher melting or boiling point is shown in red.

You will notice that:
the trans isomer has the higher melting point WHILE
the cis isomer has the higher boiling point.