We use electron holography to profile the local internal potential due to spontaneous polarization and piezoelectric effects in strained quantum well structures of wurtzitic group III nitrides. Profiles of the electrostatic potential across a GaN/InxGa1-xN/GaN quantum well structure show the existence of internal electric fields of about -2.2 ± 0.16 MV/cm, and a potential drop across the quantum well of 0.6 ± 0.16 V. The electric fields indicate an average indium composition of 15% in the quantum well, for a thickness of 2.7 nm. This indium composition compares well with measurements by energy-disperse spectroscopy of 18 ± 2 %. Screening effect is not observed under these experimental conditions.