[ghedo]

Using curl is one way:

  % curl -v https://google.com/ --http2
  *   Trying 2607:f8b0:4007:802::1008...
  * Connected to google.com (2607:f8b0:4007:802::1008) port 443 (#0)
  * found 180 certificates in /etc/ssl/certs/ca-certificates.crt
  * found 722 certificates in /etc/ssl/certs
  * ALPN, offering h2
  * ALPN, offering http/1.1
  * SSL connection using TLS1.2 / ECDHE_ECDSA_AES_128_GCM_SHA256
  ...
  * ALPN, server accepted to use h2

The above is with the GnuTLS build on Debian, recent enough OpenSSL versions support both NPN and ALPN.

[achillean]

Shodan also keeps track of the ALPN negotation results, see the "Negotiated HTTP Versions" chart: https://www.shodan.io/report/tbi6kDZa