How can we tell which layer is which during a liquid liquid extraction?

Higher density molecules are below, lower density are above. What are you separating?

I am not separating, it's just a general question about extraction.. But what if we didn't know the densities? Then how could we tell? 

Post Merge: 10 years ago

On a related note, do you know how we could separate a mixture of toluene and benzoic acid? 

HI

Make a solution of Sodium Hydroxide or Sodium Carbonate and do three to four solvent solvent extractions using the aqueous base and the mixture of benzoic acid and toluene. Because benzoic acid reacts with base to form the water soluble carboxylate ion, it will react and the dissolve in the aqueous solution. Upon separation of the two phases acidify the aqueous extract with dilute HCl, a white needle-like precipitate of benzoic acid will form when the solution becomes acidic. Simply filter the crystals, wash with cold water, and then recrystallize using a mixture of 50/50 water and methanol or some other suitable solvent.

Thank you