  We study the magnetic ordering transition for a system of harmonically
trapped ultracold fermions with repulsive interactions in a cubic optical
lattice, within a real-space extension of dynamical mean-field theory (DMFT).
Using a quantum Monte Carlo impurity solver, we establish that
antiferromagnetic correlations are signaled, at strong coupling, by an enhanced
double occupancy. This signature is directly accessible experimentally and
should be observable well above the critical temperature for long-range order.
Dimensional aspects appear less relevant than naively expected.
