  Precise control of magnetic fields is a frequent challenge encountered in
experiments with atomic quantum gases. Here we present a simple method for
performing in-situ monitoring of magnetic fields that can readily be
implemented in any quantum-gas apparatus in which a dedicated
field-stabilization approach is not possible. The method, which works by
sampling several Rabi resonances between magnetically field sensitive internal
states that are not otherwise used in a given experiment, can be integrated
with standard measurement sequences at arbitrary fields. For a condensate of
$^{87}$Rb atoms, we demonstrate the reconstruction of Gauss-level bias fields
with an accuracy of tens of microgauss and with millisecond time resolution. We
test the performance of the method using measurements of slow resonant Rabi
oscillations on a magnetic-field sensitive transition, and give an example for
its use in experiments with state-selective optical potentials.
