Knowing how to determine when the last time package index files were updated can be beneficial when you least expect it. It is not something used every day but can be used to monitor or verify when apt-get update command was executed.

The idea is straightforward as it is based on the modification time of the most recently updated package index file located in /var/lib/apt/lists/ directory.

$ TZ="UTC" date -d @$(find /var/lib/apt/lists/ -type f -not -name lock -printf '%T@\n' | sort -n | tail -1 | cut -d. -f1)
Thu Sep 15 12:50:54 UTC 2016

You can modify it to verify whether specified package index files are up to date.