Plywood is defined by being made layers of wood with the grain in different directions. So the factor that will tell you if something is ply is whether or not that definition is applicable. When it comes to comparing material properties, knowing what materials you are dealing with is a necessary first step. The fact that it seems to be a point of contention about the difference between ply and laminate shows that this information is not yet sufficiently transparent.
Density is probably not so much of a factor as stiffness, although there is a degree of relationship between the two when applied to wood commonly used in instrument construction. Stiffness only really matters in one direction since all the strings go in a single direction, so a construction with all the laminates in the same direction is going to provide superior stiffness and consistency to alternating plys.
As to maple vs rosewood, they both vary greatly in density, since there are loads of species, and variation according to position in a tree. Rosewood boards are going to be laminated into a maple neck whereas a maple board might not be, additionally a maple board will be lacquered and a rosewood one probably won't be. But Rickenbackers use a lacquered dark woo board, might be rosewood or related, and are known for bright tone, so maybe you are entirely looking at the wrong factors.