Short answer, you can't for the MCAT.
In such a reaction (HA + B --> A- + HB+), you have multiple things going on. By the way, I assumed that HA and B were both neutral, but this is arbitrary. HA first dissociates (Ka), producing some A- and H+. Similarly, B is hydrolyzed (Kb), producing some HB+ and OH-. But then H+ and OH- will react to near completion (Kw). This drives the equilibrium of both acid dissociation and base hydrolysis forward. But then the conjugate acid and the conjugate base (A- and HB+) start accumulating. There is a strict quantitative way of solving for the concentration of all the species (HA, A-, B, HB+, H+, and OH-), which involves solving a system of 6 equations, but it is unnecessary for me to explain this process.
Qualitatively, equilibrium will favor the side with the weaker acid and the weaker base. That is, you can compare the pKa of HA and the pKa of HB+. The side with the higher values (lower strength) will be favored.
For example, assume we have weak acid dissociation, where water acts as base (H2O = B). Since HA is a weak acid, we can assume its pKa is at least 0. The pKa of the conjugate acid, HB+ (H3O+) is -1.7, so that side is not favored. You can do the same thing for strong acid + weak base, and you will see that the reaction equilibrium lies far to the right (favors the conjugate base and the conjugate acid). The same applies for strong base + weak acid. But weak acid + weak base requires the actual pKa and pKb values.
