  We show that the excitation of long-range Rydberg molecules in a
three-dimensional optical lattice can be used as a position- and time-sensitive
probe of the site occupancy in the system. To this end, we detect the ions
which are continuously generated by the decay of the formed Rydberg molecules.
While a superfluid gas shows molecule formation for all parameters, a Mott
insulator with $n=1$ filling reveals a strong suppression of the number of
formed molecules. In the limit of weak probing, the technique can be used to
probe the superfluid to Mott-insulator transition in real-time. Our method can
be extended to higher fillings and has various applications for the real-time
diagnosis and manipulation of ultracold lattice gases.
