1) recessions typically have increasingly short life spans (historically). The length of recessions has been decreasing historically on average (while expansion durations have been increasing).
2) they do not always end the same way.
There is no exact measurement to gauge the end in hindsight, however, generally, low interest rates, pumping money into the economy, and forward optimism (often war as well) seemed to be common elements to the end of prior recessions.