By using a checkpoint table in the OGG replicat database, you can retrospectively review the replicat's lag using flashback query (with some limitations if you do not utilise a heart beat table).
Picture the scenario. You receive observations that the Oracle GoldenGate (OGG) replicat system that you are responsible for had fallen behind overnight and all overnight processing was performed against incorrect data. What do you do? Perhaps the observation was erroneous. How can you demonstrate this?