What's the best way to determine if a managed account is stale?

I want to disable or delete managed user accounts if they are inactive for 90 days.  What is the best way to determine this?

The reports that I have ran provide some useful info, but I really just need to know WHEN a user last logged on.  Other reports show date ranges, but only show users that have logged on during that time frame.

Is there no simple answer to "when did this user last log on?"

Also - the box Drive applet makes it challenging because users that don't log on via the website, and only use box Drive don't always register as recently being active, or logged on - I found that out the hard way!