A good mortar is one, which is cohesive and have adequate workability in it.
Different sand requires different proportion of mortar.
You need to ensure the silt in sand should be less than 8% by volume and less than 3% by weight.
For crush sand it should be less than 5%.
Silt is particle smaller than 75 micron.
You need to do some trails with different combination and should check its
Stickyness on wall
Resistance to cracking
If required do pull out test on samples to know the tensile adhesion.
Tensile adhesion will be depend on the base material properties too, on which plaster is applied.
For brick work - there is not much you can do other than cohesiveness and workability.
For waterproofing - most important parameter is, it should not carck. It means do check silt in sand.