The spatial distribution of the ion cyclotron orbit 'guiding centers' (magnetron radii) in a Penning trap determines virtually all aspects of Fourier transform ion cyclotron resonance (FTICR) mass spectrometry performance. Here, we demonstrate experimentally a simple method for determining that distribution, based on measuring FTICR mass spectral peak height following dipolar excitation/dipolar detection at various stages of magnetron radial expansion. No instrumental modifications are needed. Such data will make it possible to evaluate theoretical models of space charge and to quantitate and optimize ion formation, ion injection, ion trapping, ion transfer, photodissociation and ultrahigh-resolution FTICR experiments.