@maxnb The easiest way to test for it that I've seen is to open the site in safari, which in the network tab of the inspector will show HTTP/2.0 200 on an asset (or page) loaded successfully via http/2.
There's an extension for Chrome that makes it easy to see.. I've not seen it clearly indicated within the inspector though.
Just do a curl -I theURL (that's a capital i). It will tell you the http value. I can say that all 6 of my DO Forge servers (all recent) are still serving http/1.1 even though Forge configured http/2.0 out of the box. But, I think @willvincent stated the best way right now. I get varying results with cURL and more consistent with HTTPie. New Safari seems the most consistent.