Note: HTTP/2 and SPDY are not used for the connections between Cloudflare and the Origin server. Read more here.
When a client makes a connection to your website both the web browser and server will automatically use the highest protocol available. This means that HTTP/2 takes precedence over SPDY, and SPDY takes precedence over HTTP/1.x.
To determine what protocol is used for your connection you can visit [yourdomain.com]/cdn-cgi/trace (replacing [yourdomain.com] with your domain name). You should see information similar to the example below:
fl=21f63 h=www.cloudflare.com ip=22.214.171.124 ts=1420194924.494 visit_scheme=http uag=Mozilla/5.0 (Macintosh; Intel Mac OS X 10_10_1) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/39.0.2171.95 Safari/537.36 colo=SJC http=h2 loc=US
The line http=h2 in the above example indicates that HTTP/2 is being used. Other possible values are http=spdy/3.1 for SPDY/3.1 and http=http/1.x for HTTP/1.x.