What are the best sites / tools for testing 4G proxy quality?

Thank you!

Quality in what sense?

Do you want to check if all your provider's 4g proxies (the ones you get access to) are all IPs from mobile carriers?

Or do you want to know how fast they are?

Or on what website can they be used without any issues?

If you can narrow your question, you can get a better answer.

I agree with the post above. It is better not to write a short question in the title of the topic, but to describe its essence in the topic itself.

As for the question, I personally haven't heard of proxy-checkers for 4g proxies