When syncing via WLAN, the most recent sync date/time is displayed on the iOS device but not on iTunes (where only the time of the last backup is shown which seems to be the time of the most recent cable-based sync). Is there a way to show the most recent WLAN sync as well?
On iTunes select your iDevice from left menu and
Summary tab on the right pane, under
Backup section you'll see a sentence like
last backed up to this computer: Today 06:30 this date shows you the most recent sync, whether it was over WiFi or USB.
Here is mine: