PCl5 (g) <=> PCl3 (g) +Cl2

Question

what is the property that might be observed in order to determine when equlibrium has been reached?

The question is a little vague. Is the property visual or some other property? I don't think vision (color) will work for PCl5 is a yellowish color and chlorine gas is ayellow green. But I wonder about the possibility of noting that 1 mole of PCl5 produces 2 mols of gas; therefore, if the PCl5 is placed in a sealed container (at a constant temperature) that had a pressusre guage attached, I wonder if equilibrium for the reaction would be noted by the pressure not changing? Check my thinking.