openssl s_client -connect example.com:443 -quiet prints certificate chain, you'll see something like this - verify return:1 depth=1 C = US, O = Let'

openssl s_client -connect example.com:443 -quiet prints certificate chain, you'll see something like this - verify return:1 depth=1 C = US, O = Let's Encrypt, CN = Let's Encrypt Authority X1 - meaning Let's Encrypt is involved.