How to tell what user has synced
Not really anything obvious.

The easiest would be if you have a backup you could restore to view it.

Otherwise you'll properly try - either yourself, daunting, or via commercial tools to find a tool that lets you explore the transaction log for the given changes.
For example something like: http://www.apexsql.com/sql_tools_log.aspx

I've not tried out such tools myself so I cannot vouch for them. 
I usually get by either by having backups to search, or in some cases, I've set up change tables to log changes to critical data.