  We present a viable Monte Carlo determination of the scaling dimensions
$\Delta_Q$ of flux $Q$ Abelian monopoles through finite-size scaling analysis
of the free energy to introduce the background field of classical Dirac
monopole-antimonopole pair at critical points of three-dimensional lattice
theories. We validate the method in free fermion theory, and by verifying the
particle-vortex duality between the monopole scaling dimension at the
inverse-XY fixed point and the charge scaling dimension at the XY fixed point.
At the $O(2)$ Wilson-Fisher fixed point, we determine the critical exponents
$\Delta_1= 0.13(2)$, $\Delta_2=0.29(1)$ and $\Delta_3=0.47(2)$, which we find
to be proportional to the finite-size critical spectrum of monopoles on square
torus.
