Ultracold atoms in optical lattices have rapidly become a favoured tool to simulate and test fundamental concepts of condensed matter physics, in particular to simulate electrons in solid crystals. Recent experiments with an optical resolution of single atoms at individual lattice sites have resulted in the direct observation of quantum phase transitions, such as the celebrated superfluid to Mott insulator transition for bosonic particles. However, an experimental proof of single-site-resolved detection of correlated phases of ultracold fermions in a lattice is still missing.