Layer deposition of organometal halide perovskites for solar cells usually involves tedious experimentation, in which numerous devices are tested to determine the ideal processing conditions. One of the important issues is determining the optimum time and temperature for thermal annealing of the perovskite layer. Here we demonstrate that in-situ photoluminescence allows to determine the optimal annealing procedure without fabricating complete solar cells. We use a deposition method in which dense layers of perovskite crystals are formed within seconds in ambient air by hot casting a mixture of lead acetate, lead chloride, and methylammonium iodide. The as-cast perovskite layers are highly luminescent because charge carriers are unable to reach the charge extraction layers that quench the photoluminescence. Thermal annealing enhances charge transport and quenches the photoluminescence, but deteriorates the photovoltaic performance via decomposition of the perovskite if applied for a too long time. We demonstrate that the optimal annealing time coincides with the time required for the in-situ measured photoluminescence intensity to reach its baseline value for annealing temperatures in the range of 80-100 °C. This results in efficient (>14%) perovskite solar cells and shows that in-situ photoluminescence is a simple but powerful tool for in-line quality monitoring of perovskite films.