This test site: https://tls18.flynexity.com/
I do not have access to this site … I can see it through my browser …
How can I find out which “challenge” it uses? (http-01 or tls-sni-01)?
You can't as long as you don't have access to the issuance process.
… ok bytecamp… thanks!..
Although @bytecamp's answer is correct in principle, we can rule out the possibility of TLS-SNI-01 in this case because the first certificate for that name was issued on January 22, and TLS-SNI-01 had already been disabled for new certificates before then.
So, it would have to have been one of the other methods, either HTTP-01 or DNS-01.
It's true that information about the challenges that were used to obtain a particular certificate isn't published by Let's Encrypt, although I believe Let's Encrypt policies allow information like this to be published for security research purposes in the future if it's decided that this would be beneficial.