Cloudflare does have Passive Origin Monitoring you can enable within the Dashboard under Notifications, and I think they might also email automatically about specific error conditions like 500 errors? But those emails would be sent in the past, and require requests to be flowing through your zone.
I think your best bet might be going to Analytics → Status Codes, selecting the past 30 days and seeing when error codes like 523/524 spike. Here’s a magic link: https://dash.cloudflare.com/?to=/:account/:zone/analytics/traffic. You can do this on free analytics. If you have Pro, you can revert (bottom left hand corner) to see the old style analytics with up to 30 days of history. This would only work if you have traffic though.
Other then that, if you have access to your server you could check its logs as for when it shut off/booted up last, or nginx/apache logs for the last request.
I would recommend some proper monitoring in the future, there are a lot of free options like HetrixTools/Uptimerobot, and Cloudflare offers Health Checks for Pro and above. As discussed above, you can also enable Passive Origin Monitoring within Cloudflare, but that would only trigger if people are using your site and Cloudflare detects it cannot reach out, it’s not active monitoring.
Lastly… you could just ask your host.