The best way to get quality mortar on the job is by inspection. Testing has historically caused more controversy in mortar quality than it has provided answers.
I am a proponent of using C780, as there are numerous issues with using C270 as a field testing method. I also feel that compressive strength is an appropriate acceptance parameter for mortar (contrary to the mortar cement suppliers!), provided the testing is properly done (using 3"x6" molds, field protection, proper laboratory testing, etc.
Proportion specifications will work, but they don't correlate consistently with compressive strength. Though safety factors can be high in masonry design, as an Engineer I prefer quantification of mechanical properties, not just warm, fuzzy feelings!