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In my opinion inspection during mixing, ensuring that the specified proportions are followed, is the best thing to do. There is also a field test (you can find it on internet- field test from ASTM C780) using methyl alcohol by which you can find the ratio cement/sand of the tested mortar. Of course the mortar quality depends also on the raw material's quality. Plenty available tests for them (even for the water) which obviously cannot be carried out on site. My opinion: visual inspection of sand (particle size and clay existence- up to a certain point they can be noticed without tests), visual inspection of mortar (poor/ rich in cement) and when it's possible inspection during mixing...at least that's what I did about 15 years ago when I had the same question...don't worry, my walls are still standing...
