Hi,
It is we as the designer should know it.According to my knowledge i dont think there is any tool options to check about blockage layer.
I can give an instance where to use the blockage layers.
If certain inputs or the outputs have to be accesses only by the top metal layers then we need to add blockage to the lower metal layers.In some schematics they will be having 2 different supply voltages there we need to see that the power signals connected by lower level metals are blocked using a blockage layer and the top metal layer is used for accessing.