From the topic title:
Best way, self test, or test by asking other users for feedback information.
Otherway is to test using some online tools like:
- PageSpeed Insights
- Bulk Page Speed Test - Check Your Website Speed | EXPERTE.com
If you are interested into using Cloudflare for your domain/Website, you would need to create Cloudflare account and add it to your account, them change the nameservers and fine-tune per-need.
Kindly, see the below article how to get started for free
Depending on the end-user location, he/she should notice the differences regarding TTFB time and other. There are a lot of options at Cloudflare for page speed optimization at the Cloudflare dashboard like image optimization, HTML, CSS and JS minification, browser cache.