Each Proposal, Invoice and PO that should be connected to QBO will have a QB button next to the Document Number.
If green, this document is successfully Synced.
If the QB button is red, this means there was an error with sync. Clicking the red button will run a sync error diagnostic as well as give you the chance to retry.
Based on the nature of the error, the sync diagnostic may give you the option to automate an action on QBO in order to allow for a successful sync.
See video below.