It should be in the Marketo Error Log in CRM. It should be under settings, wherever that is in you particular edition.
Thanx Kenny, I found those. Any idea if there is any error messaging in Marketo regarding errors? it seems sync happens very rocky (sometimes a newly create contact syncs, sometimes it doesn't).
Certain classes of errors will appear in your Marketo notifications, but I don't really know if there's a list of which will appear where.