RE: How do we know they used it?
Well, all those warnings by the Russkies that they will shoot down any missiles fired on Syria are baloney then.
See here for one:
https://nypost.com/2018/04/11/russia-warns-any-us-missiles-fired-at-syria-will-be-shot-down/
To get real assessments of national security policy and the way it works you have to understand what disinformation and how/why saber rattling is done. There is the version the press reports and then there is the truth which is conducted at a need to know basis.