How to remove synced iPhone data in iTunes 11?
There isn't an easy way to have iTunes show you the last sync data.

You could use Time Machine to compare the files in ~/Music/iTunes after the sync with before it. (Or just restore that entire folder or the entire Mac from a backup before the sync.)

The automatic sync that happens with a new computer is not any actual transfer of files as you would have to go into iTunes and set up the rules or accept the automatic sync of content so unless you already had synced that device to that iTunes library before no data would transfer.