How do I determine which app is syncing?
When you go in to accounts and sync. It is going to show which account is currently syncing. Then from there, you can go in to the account and you will see what type of sync is happening. 

It will show, for example, that "Reader" is syncing. So Google Reader is the App that is syncing. Or it will Show "Gmail" is syncing.

Those would be the apps that are syncing. I don't think just any app can trigger a sync for some other app. Like Reader shouldn't be able to trigger a sync for Picasa Images. But really, even if they can trigger other apps to sync, it is still the original app that is performing the sync. Only the twitter app is going to sync "twitter data and contacts", and only the Gmail app is going to sync your Gmail.