If you are not in Mainland China using the infrastructure there (not a foreign SIM card), then it’s hard to test your website’s speed.
Fortunately, there are websites that help with that. They aren’t always 100% accurate, but can be used as a good indicator.
The one I personally like best is WebPageTest.
Here’s how to use it to test your site in Mainland China:
Testing your website in Mainland China
First go to WebPageTest.

Put in your URL and click on “Advanced Configuration”.

For “Test Location” pick either Shanghai, Beijing or Ningxia. Then click on “Start Test”.
Wait for the result
Waiting to get a result might take some time. You might have to queue, and even when your test starts, be prepared to wait another 10 minutes to see a result.
Your screen will look something like this for some time:

Results
After some time, you’ll see results. They will look something like this:

Interpreting those results is the secret sauce. If your loading times are between 5-15 seconds, you should be fine.
What you want to look out for are very long lines, indicating that certain files get loaded very slowly.
Limitations
This tool has its limitations in that it not always accurately mirrors the real world when it comes to e.g. Google Fonts or Gstatic files. They usually seem to load reasonably fast here, which they aren’t always in the real world.
However, it’s a great tool to get a rough handle on how your website is performing in Mainland China.