We demonstrate that the probability distribution of the net number of electrons passing through a quantum system in a junction obeys a steady-state fluctuation theorem (FT) which can be tested experimentally by the full counting statistics (FCS) of electrons crossing the lead-system interface. The FCS is calculated using a many-body quantum master equation (QME) combined with a Liouville space generating function (GF) formalism. For a model of two coupled quantum dots, we show that the FT becomes valid for long binning times and provide an estimate for the finite-time deviations. We also demonstrate that the Mandel (or Fano) parameter associated with the incoming or outgoing electron transfers show subpoissonian (antibunching) statistics. ; Comment: 20 pages, 12 figures, accepted in Phy.Rev.B