Where can I monitor if emails are being quarantined or rejected?

If you've set up a quarantine DMARC policy, this means that you're asking receivers to send messages to spam that fail DMARC. If you have a reject policy in place, you're asking receivers to completely block delivery of the messages that fail DMARC. 

In DMARC Digests, if you click on a domain, you'll be able to see the number of emails reported in a certain time period and how receivers handled specifically those messages:

Note that the DMARC reports we collect only include aggregate data, so it’s not possible to see more detailed information about specific messages which were quarantined or rejected.