We demonstrate an all-optical approach to probe electronic band structure at buried interfaces involving polar semiconductors. Femtosecond optical pulses excite coherentphonons in epitaxial GaPfilms grown on Si(001) substrate. We find that the coherentphonon amplitude critically depends on the film growth conditions, specifically in the presence of antiphase domains, which are independently characterized by transmission electron microscopy. We determine the Fermi levels at the buried interface of GaP/Si from the coherentphonon amplitudes and demonstrate that the internal electric fields are created in the nominally undoped GaPfilms as well as the Si substrates, possibly due to the carrier trapping at the antiphase boundaries and/or at the interface.