How is it possible to predict looking on the stuctural formula of a two molocules to preict which one is more accidic/basic? It depends on the compounds, the definition of acids and bases you're using, and under what conditions. Yes that's vague, but without more information to go on, that's about as accurately as that question can be answered. There are numerous definitions of acids and bases: Arrhenius, Brønsted-Lowry, and Lewis acid/base theories are just a few that ar out there. All three are pertinant to orgnaic chemistry, but there are plenty of compounds that are defined as acidic or basic by one theory, but not another.
For example, using the Brønsted-Lowry definition of acids and bases, an acid is a species that donates a proton (that is, it donates a H+), and a base is one that accepts it. The acidity of a compound is determined by its pKa, or the dissociation constant - the lower the constant, the more acidic it is.
So, how do you tell how acidic an organic compound is? Look at its structure, and look for functional groups that will cause a proton to dissociate. Once the proton is dissociated, the compound will be ionic, so also look for functional groups that will stablize the deprotonated form.
I'm guessing you want more information than this, but unless you're willing to be more specific about the compounds you're interested in, this could take a while...