While it’s not specific to MP6 (I am not following its development) and its CSS, to me your question sounds like “how to check that WP version is equal to greater than one some feature was introduced in?”.
So I would just examine content of $wp_version global. If it’s 3.8 feature then anything with fitting version has it.
Also from quick look at body classes in admin there is branch-3-7 class, which makes possible to target branch-3-8 (however cannot be targeted conditionally as anything >=3.8 since that’s not something CSS does).