Unfortunately there is not a way to determine when the year-end function was performed. The only way to revert back from a year-end close is to restore from a backup prior to the year-end. This is why it is highly advised that a backup is taken before doing the year. In fact when performing the Y/E function there is a reminder to perform a backup.

Actually, there is a text file that is created when a Year-End is done in BV. Check your company data folder and look for a file such as yyyyYearEndLog.txt. In here you will see the date that the year-end was closed and all the relevant tables that were changed.